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Evidence in Medicine: Correlation and Causation

There are two general approaches to subverting science-based medicine (SBM): anti-science and pseudoscience. Anti-scientific approaches are any that seek to undermine science as the determinant of the standard of care, often overtly advocating for spiritual or subjectively-based standards. Some attack the validity of science itself, usually with post-modernist philosophy.

Pseudoscientific proponents, on the other hand, praise science, they just do it wrong. In reality there is a continuum along a spectrum from complete pseudoscience to pristine science, and no clear demarcation in the middle. Individual studies vary along this spectrum as well – there are different kinds of evidence, each with its own strengths and weaknesses, and there are no perfect studies. Further, when evaluating any question in medicine, the literature (the totality of all those individual studies) rarely points uniformly to a single answer.

These multiple overlapping continua of scientific quality create the potential to make just about any claim seem scientific simply by how the evidence is interpreted. Also, even a modest bias can lead to emphasizing certain pieces of evidence over others, leading to conclusions which seem scientific but are unreliable. Also, proponents can easily begin with a desired conclusion, and then back fill the evidence to suit their needs (rather than allowing the evidence to lead them to a conclusion).

For example, the anti-vaccine movement systematically endorses any piece of evidence that seems to support the conclusion that there is some correlation between vaccines and neurological injury. Meanwhile, they find ways to dismiss any evidence which fails to show such a connection. They, of course, accuse the scientific community of doing the same thing, and each side cites biases and conflicts in the other to explain the discrepancy. It is no wonder the public is confused.

How, then, do we use the evidence to arrive at reliable scientific conclusions? That is what I will be discussing in this series of posts, beginning with a discussion of correlation and causation, but here is a quick overview: SBM is achieved through a consideration of scientific plausibility and a systematic review of the clinical evidence. In other words – all scientific evidence is considered in a fair and thorough manner, including basic science and clinical evidence, and placed in the context of what we know about how the world works. Further, we do not rely upon any individual’s systematic review of the evidence, but on the consensus of analysis among experts and institutions – so that any biases are likely to average out.

This leads us to the final continuum – the consensus of expert opinion based upon systematic reviews can either result in a solid and confident unanimous opinion, a reliable opinion with serious minority objections, a genuine controversy with no objective resolution, or simply the conclusion that we currently lack sufficient evidence and do not know the answer. It can also lead, of course, to a solid consensus of expert opinion combined with a fake controversy manufactured by a group driven by ideology or greed and not science. The tobacco industry’s campaign of doubt against the conclusion that smoking is a risk factor for lung cancer is one example. The anti-vaccine movement’s fear-mongering about vaccines and autism is another.

Correlation and Causation

Much of scientific evidence is based upon a correlation of variables – they tend to occur together. Scientists are careful to point out that correlation does not necessarily mean causation. The assumption that A causes B simply because A correlates with B is a logical fallacy – it is not a legitimate form of argument. However, sometimes people commit the opposite fallacy – dismissing correlation entirely, as if it does not imply causation. This would dismiss a large swath of important scientific evidence.

For example, the tobacco industry abused this fallacy to argue that simply because smoking correlates with lung cancer that does not mean that smoking causes lung cancer. The simple correlation is not enough to arrive at a conclusion of causation, but multiple correlations all triangulating on the conclusion that smoking causes lung cancer, combined with biological plausibility, does.

Correlation must always be put into perspective. There are two basic kinds of clinical scientific studies that may provide evidence of correlation – observational and experimental. Experimental studies are ones in which some intervention is given to a study population. In experimental studies it is possible to control for many variables, and even reasonably isolate the variable of interest, and so correlation is a well-designed experimental study is very powerful, and we generally can assume cause and effect. If active treatment vs placebo correlates with a better outcome, then we interpret that as the treatment causing the improved outcome. (“Well-designed” is the key here – a subject of a future post.)

In observational studies populations are observed in the real world, but no intervention is being given. Observational studies can be very powerful, because they can look as extremely large numbers of subjects (more than is practical in an experimental study) but the weakness is that all variables cannot be controlled for. Researchers can account for known variables (race, age, and sex are common), but it is always the unknown variables that can confound such studies.

In observational studies lack of correlation is easier to interpret than a positive correlation – if there is no correlation between A and B then we can pretty much rule out a causal relationship. The only caveat is that a correlation is being obscured by a factor that was not accounted for. When a correlation is found in observational studies – that is when the assumption of cause and effect must be avoided, and more thorough analysis is required.

If A correlates with B, then A may cause B, B may cause A, A and B may be caused by a common variable C, or the correlation may be a statistical fluke and not “real”. Further studies are then required to confirm the correlation and any specific causal hypothesis.

To use the smoking example again – the hypothesis that smoking causes cancer as the causal relationship to explain the correlation raises several predictions, all later confirmed. The duration of smoking increases risk of cancer (a dose response relationship), stopping smoking reduces the risk of cancer, greater intensity of smoking increases risk, and smoking unfiltered vs filtered cigarettes is associated with higher risk. These various correlations only make sense if smoking causes lung cancer. Further, tobacco smoke contains substances demonstrated to be carcinogens – so there is biological plausibility.

The greatest abuse of the correlation equals causation fallacy is the assumption of cause and effect from a single anecdotal case. Here we are not talking about an observational study where statistics are brought to bear on hundreds or thousands of subjects, but the uncontrolled observation of a single individual. Such cases are very compelling to the human psyche- we are more moved by stories than statistics. But they make for very weak scientific evidence. This is not to say they are worthless – even a single case can raise the question of a possible correlation. But they cannot be used to establish even that a correlation is real. (Anecdotes generate questions, not answers.)

Again to use the anti-vaccine movement as an example, it is easy to generate fear based upon individual cases of bad outcomes after receiving the vaccine. We are hard-wired to find such events compelling. But such correlations should and do occur on a regular basis, even without any causal factor. Further, it is natural, after a new disease or disorder appears, to think back over any recent events that may explain it. Our minds will latch onto anything that sticks out, and over time our memories will even morph to make the apparent correlation more compelling.

Here is an anti-vaccine and conspiracy site that is essentially collecting anecdotes of correlation between the flu vaccine and miscarriages among women. This goes beyond the assumption of cause and effect from correlation, to the assumption of correlation from anecdote. Common things occur together commonly. Given the number of spontaneous miscarriages, and the number of pregnant women receiving the flu vaccine, we would expect there to be thousands of women who miscarry within 24 hours of receiving the flu vaccine, just be chance alone. So first we have to ask – is this a real correlation?

The answer, according to systematic reviews of existing evidence, is no. There is no apparent risk of adverse outcome in pregnancy from the flu vaccine.

The media tends to develop a narrative they think will sell, and then that becomes the story they are telling. In the midst of this severe flu season, the narrative the media is telling is of dramatic adverse events following the flu vaccine. These events are always there, because people are always getting sick, and when you vaccinate millions of people, some of them will get sick afterwards by chance alone. Individual stories are therefore misleading – we need statistics on large number of people to arrive at any conclusions. But statistics don’t make headlines – individual stories do.

The CDC and the WHO are tracking the statistics as the seasonal and H1N1 flu vaccines are rolled out. In the end, these statistics will tell the story.

In conclusion, correlation is an extremely valuable type of scientific evidence in medicine. But first correlations must be confirmed as real, and then every possible causational relationship must be systematically explored. In the end correlation can be used as powerful evidence for a cause and effect relationship between a treatment and benefit, or a risk factor and a disease. But it is also one of the most abused types of evidence, because it is easy and even tempting to come to premature conclusions based upon the preliminary appearance of a correlation.

Posted in: Science and Medicine

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39 thoughts on “Evidence in Medicine: Correlation and Causation

  1. Steven – Great post. I feel motivated to add a dimension. Science does not provide answers. It provides a process for increasingly arriving at ever-more defensible answers. This is subtle, but crucial. It is a myth held by some, hopefully not you, that science arrives at knowledge. It does not. Asymptote, not Arrive. Science is a process, and always has doubt at its heart. For “answers,” you need to take a jump from science to opinion, belief, conclusion, whatever you might want to call it. This is subtle, and I have noted a couple examples here, where it is operational in your post…

    “How, then, do we use the evidence to arrive at reliable scientific conclusions?”
    Exactly. Science does not provide the conclusions, but can help make conclusions scientific. We take the products of the scientific effort, and we impose an explanation, we impose some coherent framework. We add a conclusion.

    “If active treatment vs placebo correlates with a better outcome, then we interpret…”
    That’s the spirit! We take observatons from science, and we put the final interpretation on the evidence on hand. If another person comes by with a better interpretation for the given data, logic and reason may win us over, with the same handful of scientifically generated evidence.

    Science provides a steady, reliable process for increasingly approaching ever-more defensible hypotheses regarding reality. We have to take the various observations from scientific endeavors and we have to interpret these. Science does not interpret; does not impose a coherence upon a scatter of observations, whether from an experiment or from observations.

    “…and placed in the context of what we know…”
    Yes, we put info into contexts. Science does not. The process of science must be combined with our reason and cleverness to put various observations into a coherent context. Science does not do that; we do that with our minds and imaginations. Science just provides a robust self-improvement process. We could use other processes to arrive at the same conclusion. The conclusion would not be science-based. If the conclusion were about medicine, the conclusion would not be science-based medicine. If we do rely heavily on science to support a belief or opinion, such as what causes what, or what cures what, then we are in the realm of science-based medicine.

    I just felt motivated to bring up the topic of whether science actually answers questions, or not. We speak that way casually, and I can handle that. However, by scientific orthodoxy, I believe science, itself, does not answer questions. This recognition actually helps me as I approach a health care issue where there is conflicting or otherwise varied data (doesn’t it always seem to be the case?). If you disagree, if you believe science answers questions, I would be interested in hearing your view on this.

  2. Med – I agree that science is a process, not a destination. But I think you got a bit too caught up in semantics there.

    Saying a “reliable scientific conclusion” covers it, if you include in the definition of “scientific” a tentative process. And no conclusion is final or absolute, just reliable.

    The article was also written in the context of science-based medicine (which I assumed was obvious, but is always worth pointing out). Which means we are not talking about coming up with final answers about how the world works, but applying what we know to the practice of medicine. So the “conclusions” I am talking about are whether or not we should treat disease A with treatment B.

    In medicine we have to make decisions in the context of imperfect and tentative knowledge. All we can do is use a process that makes those decisions as reliable as possible, and as scientific as possible – reliable scientific conclusions.

    In fact I use that phrase specifically to account for the points you raise.

  3. Ed Whitney says:

    OK, time to move on from the easy cases to the tough cases. For the second day in a row, the evening news broadcasts lead story was the new mammography guidelines from the US Preventive Services Task Force, and the rejection of same by the American Cancer Society and other breast cancer physicians and organizations. The conclusions of the USPSTF are nearly identical to the conclusions of the NIH Consensus Development Conference in 1996, that the science does not support routine screening for women under 50. The American College of OB-GYN is sticking to its earlier recommendations in spite of the findings of the USPSTF.

    Are they guilty of pseudoscience? Are they rejecting science?

    Not always so easy to judge these things, is it?

  4. David Gorski will be publishing a full post examining the new mammography guidelines. Stay tuned.

  5. Plonit says:

    Anti-scientific approaches are any that seek to undermine science as the determinant of the standard of care…

    +++++++++++

    I think I have to take issue with this. There are surely extra-scientific concerns that impact on the standard of care, but holding these to be important in medical care is not necessarily anti-scientific.

    For example, the “standard of care” surely includes providing information, ascertaining the capacity of a patient and gaining (or not) consent for a procedure. This all belongs to the realm of philosophy and ethics, and hardly at all to “science” as usually conceived. Of course, we can study the most effective techniques for imparting information in a scientific way. But it is not science that dictates that patients ought to receive information and consent (or not).

    One might, purely hypothetically, imagine a study showing that patients receiving information and able to consent have worse outcomes (by some objective measure) than patients who are told nothing and from whom consent is not gained. This would not invalidate the claim that patients should receive information etc…, since that claim is an ethical one, not a scientific-evidential one.

    (Of course, one can seek to undermine the ethical imperative for autonomy by appealing to other, conflicting, ethical imperatives (benefience, non-malefience, social justice/equality). Or alternatively, by way of anthropology, though appealing to other cultural norms to undermine the presumed universalism of our own post-enlightenment high valuation of autonomy. Notice that the entire debate is extra-scientific though).

    Talk of objective outcome measure also raises the issue of how we go about deciding which outcome measures are the most important. This is essentially a subjective issue, albeit that there is significant cultural consensus. If you were to argue that length of survival was a more important outcome measure than “quality of life” (or vice versa), how important a role would the sciences (as traditionally conceived) play in your argument? This is not at all an esoteric discussion when it comes to treating terminal illness.

    Acknowledging that there is an important extra-scientific dimension to healthcare is not anti-science, and need not undermine science.

  6. Harriet Hall says:

    “Anecdotes generate questions, not answers.” This quotation is a keeper!

    As for the extra-scientific considerations, they are important but secondary. Science has to come up with the information before society and individuals can decide how to use the information.

  7. Plonit – I agree with your points. I was referring to the standard of care in the context of what works and what doesn’t. There is a separate ethical standard of care, which includes personal autonomy and the right to decide what outcomes are important.

    I should have clarified – but in the context of my article, I was referring only to using science to determine which interventions are safe and effective.

    Advocating for something other than science to determine safety and efficacy (not ethics or priorities) is indeed anti-scientific.

  8. Plonit says:

    Advocating for something other than science to determine safety and efficacy (not ethics or priorities) is indeed anti-scientific.

    +++++++++++

    I totally agree on that.

    It is important to undestand that decisions about what outcomes are important are not only operating at the individual level (under the rubric “personal autonomy”). More importantly, they operate at the level of clinical trials.

  9. bmcnutt says:

    Steve,

    Loved the post. I teach AP Statistics and we just finished Chapters on Correlation and Experimental Design so they will be reading your post tomorrow. I use a ton of the info I hear on SGU, and read in SBM as teaching aids and this will be another great asset.

    Thanks,

    Bruce in New Jersey

  10. Scott says:

    David Gorski will be publishing a full post examining the new mammography guidelines. Stay tuned.

    Why do I suspect I’ve already read it? ;)

  11. Plonit says:

    @ Harriet Hall

    As for the extra-scientific considerations, they are important but secondary. Science has to come up with the information before society and individuals can decide how to use the information.

    ++++++++++++++++

    I would have to disagree with this.

    For both good and ill, extra-scientific considerations influence the knowledge science is seeking. Moreoever, when purely scientific considerations (e.g. the ease of measuring a particular outcome) dictate the information sought by science, the result may be less useful than if extra-scientific considerations (e.g. what outcomes do patients regard as important) had guided the research question.

  12. Harriet Hall says:

    plonit,

    Point well taken. But “what outcomes patients regard as important” are part of good science in the first place. Instead of measuring changes in blood tests, good science measures changes in morbidity, mortality, and quality of life. We call it POEMS – patient-oriented evidence that matters.

  13. FelixO says:

    Great post!

    One typo:

    “correlation IS a well-designed experimental study is very powerful”

    should be: in

  14. IndianaFran says:

    Dr: Novella:

    This is an excellent overview introducing the principles of a science-based approach to evidence review.
    I can’t help but notice the contrast between these principles and the approach of your newest contributor.

    Exhibit A; Your commentary on correlation in observational studies
    <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

    Exhibit B: From Dr Tuteur’s post of 11/05 http://www.sciencebasedmedicine.org/?p=2392

    <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<

    In 2003 the US standard birth certificate form was revised to include place of birth and attendant at birth. In both the 2003 and 2004 Linked Birth Infant Death Statistics, mention was made of this data, but it was not included in the reports. Now the CDC has made the entire dataset available for review and the statistics for homebirth are quite remarkable. Homebirth increases the risk of neonatal death to double or triple the neonatal death rate at hospital birth.

    >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

    Dr Tuteur uses the correlation in this chart to draw a conclusion about causality (“Homebirth increases….”), and in order to do this, she ignores the fact that the cohorts compared can be shown to have notable differences in maternal age distribution, parity distribution, geographic distribution, and that there are several markers (high parity levels, educational attainment) which strongly suggest that the cohorts are not well-matched for socioeconomic status (which is not captured in the dataset and cannot be directly evaluated). In comments, the question “Has there been a systematic effort to identify and measure potential confounders?” has been asked several times and a direct answer has not been given.

    Do you feel that this data presentation is a good example of SBM?

    Exhibit C: Your commentary on evidence review:

    <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

    Exhibit D: more text from the same post on 11/05

    <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

    This is far from a fair and thorough systematic review. The conclusion that Dr Tuteur draws from the Johnson/Daviss study is not the same as the conclusion of the original researchers. While it is perfectly reasonable to reexamine a study and reach an alternate conclusion, it is barely ethical to present this conclusion without at least noting that it is, in fact, a dissent. Similarly, the implication that “all existing scientific studies” support the same conclusion cannot in any way be said to be a consensus of experts.
    Differing views have been expressed by the American Public Health Association(“Recognizing the evidence that births to healthy mothers, who are not considered at medical risk after comprehensive screening by trained professionals, can occur safely in various settings, including out-of-hospital birth centers and homes”)

    http://www.apha.org/advocacy/policy/policysearch/default.htm?id=242

    the American College of Nurse Midwives (“Despite a plethora of published papers on related topics, there are very few studies that isolate DEM practice as a variable. The outcomes documented in the literature are mixed, with some investigators reporting better than average outcomes among women attended by direct entry midwives, while others document better outcomes provided by physicians and/or nurse-midwives. It is difficult to draw a conclusion from these studies”)
    http://www.acnm.org/siteFiles/education/Direct_Entry_Midwifery_Nov_05.pdf

    the Cochrane Collaboration (“No strong evidence about the benefits and safety of planned home birth compared to planned hospital birth for low-risk pregnant women.“) http://www.cochrane.org/reviews/en/ab000352.html

    While not all of the evidence base is directly applicable to the US population and US midwifery practice, it is in my opinion misleading to imply that a settled consensus exists on this topic.

    Do you think that Dr Tuteur’s evidence summary follows the principles of SBM?

    At least one reader here feels that Dr Tuteur’s post of 11/05 is actually an excellent example of this part of your post:

    <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

    Perhaps Dr Novella can help to dissipate the dissonance here?

  15. Zoe237 says:

    “This leads us to the final continuum – the consensus of expert opinion based upon systematic reviews can either result in a solid and confident unanimous opinion, a reliable opinion with serious minority objections, a genuine controversy with no objective resolution, or simply the conclusion that we currently lack sufficient evidence and do not know the answer.”

    Yep, which is why this is so difficult for us laypeople, and if often seems to be the last one (with lots of people claiming they actually do know). I was a sixth grade science teacher for awhile and a big part of my job was teaching kids how to think scientifically, the scientific method, good research, and how to tell the difference between pseudoscience and real science. I considered that far more important than the details of the rain cycle or the solar system. Correlation versus causation was often a source of confusion. At what point does strong correlation become a likely causation, such as in the case of lung cancer and smoking? In most cases, these answers are never easy. And I liked the anecdotes raise questions, not answers quote as well.

    However, I am very skeptical of any claims (in general, not necessarily in this post) that science is a source of knowledge available to only a privledged few, and the rest of us just have to trust the experts. Along with that is the instinct mentioned above for some to assume that there is a single truth rather than better approximations of the truth, or an extension of science into multiple arenas that may not be science (like, uh, “political” science lol). The fact that everybody wants to push their ideas in the name of science, or take over the word, tells us of the special hold that science as a discipline has over us.

    Right now, however, the science people are losing and the “back to nature” people are winning (if there is a competition, which apparently there is). This will reverse itself, of course, as the pendulum swings the other way, as it always does. The internet, which I consider 100% one of the greatest advances in human history in terms of allowing ordinary people to access information, has accelerated this, somewhat counter intuitively. It is difficult to tell what is reliable information and what is not. People fall prey to to stories over statistics all the time. Eventually, we throw up our hands and give in because there is just no way to sort out all of the information. I’m not sure what the answer is, but I am sure it is not that the amateur should just trust scientists and doctors, who certainly sometimes have their own biases.

    Another good article on anecdotes:

    It’s good to talk
    The power of stories over statistics
    Thomas B Newman,

    http://www.bmj.com/cgi/content/full/327/7429/1424

  16. IndianaFran,

    As Dr. Novella wrote:

    “However, sometimes people commit the opposite fallacy – dismissing correlation entirely, as if it does not imply causation. This would dismiss a large swath of important scientific evidence.

    For example, the tobacco industry abused this fallacy to argue that simply because smoking correlates with lung cancer that does not mean that smoking causes lung cancer. The simple correlation is not enough to arrive at a conclusion of causation, but multiple correlations all triangulating on the conclusion that smoking causes lung cancer, combined with biological plausibility, does.”

    The CDC database is not the only scientific evidence that shows homebirth with an American homebirth midwife to have an increased risk of death. In fact, all the existing scientific evidence, and all the national statistics are in agreement on this.

    To my knowledge there is no evidence of any kind that shows homebirth with an American homebirth midwife to be as safe as hospital birth.

    Moreover, the idea certainly has biological plausibility. Being far away from emergency assistance in the event of an emergency is almost always associated with poorer outcomes.

    I understand that this is not what homebirth advocates tell women, but what they tell women is empirically false.

  17. Plonit says:

    Moreover, the idea certainly has biological plausibility. Being far away from emergency assistance in the event of an emergency is almost always associated with poorer outcomes.

    +++++++++++++++

    Event X may happen to members of group Y.

    The outcomes of event X are better if event X takes place close to emergency assistance (in a hospital).

    Therefore, all group Y must be hospitalised.

    This is essentially the structure of your biological plausibility argument. It is quite different from “tobacco smoke contains substances demonstrated to be carcinogens”.

    Let X be “cord prolapse” and Y be “pregnant women over 24 weeks, not yet in active labour and the presenting part not engaged.”

    I have seen a cord prolapse happen to a woman admitted to the antenatal ward for other reasons. The outcome was good despite the relatively early gestation. I have also seen a woman come in by ambulance with a cord prolapse that occured at home. That baby died. If I were to explain the different outcomes in these two anecdotes, the proximity to emergency assistance is certainly plausible.

    Yet we do not conclude that “pregnant women over 24 weeks, not yet in active labour and the presenting part not engaged” should be hospitalised. If we were to propose hospitalising this group (Group Y) we would have to weigh the benefit (proximity to emergency assistance in event X) against the individual and social risks (which for long-stay admission from 24 weeks would probably include increased DVT, depression, HAIs of all kinds, resources directed away from more important services, separating families etc…)

    I don’t know who your regard as a “homebirth advocate” but in my part of the world midwives offer women a choice of place of birth, and do explain to women that in the event of an unpredicted complication at a home birth (or in a freestanding birth centre) they will not have the immediate access to emergency assistance that is available in the hospital.

  18. Harriet Hall says:

    plonit misrepresents the plausibility argument. The available data indicate that home births in the US have poorer outcomes. What we know about emergencies and transport times makes those data plausible.

    I don’t remember anyone saying all births “should” be in the hospital. I think women have the right to deliver at home as long as they understand the risks and take reasonable precautions. I object to their being “sold” home birthing on the basis of misleading information, and I think that was the whole point of Amy’s argument.

  19. Pekka S says:

    Great post Steve! As always you have a way of making stuff like this easy to understand.

  20. Plonit says:

    plonit misrepresents the plausibility argument.

    +++++++++

    How so? The carcinogenic properties of substances in tobacco provides a plausible biological mechanism for the correlation.

    Given that that there is evidence that home birth can be a relatively safe option in some contexts (i.e. the most recent Dutch and Canadian studies) you cannot then point to a *biological mechanism* for the correlation to increased neonatal death and home birth in the US context.

    Now, you might speculate on other mechanisms specific to the US context (e.g. transfer times, as you mention). And you might also ask good questions – as IndianaFran has done – about differences in the cohorts that while you are about it.

    Why does this matter? If there is both causation and a biological mechanism for that causation, there is no incentive to change the organization of maternity care services – simply recommend that birth takes place in hospital as the inherently safer place for all women. However, if the mechanism of causation (assuming there is causation) is not biological, but social (e.g. the training of midwives, the integration of community and hospital based services, the arrangments for transfer etc…) that may dictate a different approach at a policy level.

  21. Plonit- your response to the plausibility argument for home births is a non-sequitur. The point is that the correlation of poorer outcome with home births has scientific plausibility, in that there are known factors that can lead to a cause and effect – worse access to emergency services, for example.

    You confuse this with a statement about what we do with this information – advocate for hospital births (which I think you also misstated as a mandate, but that’s a separate point).

    Also, whether or not the cause is biological or social is irrelevant to the point of plausibility. Again, you are confusing issues of remedy with the question of what cause and effect is resulting in a measured correlation.

    Looked at another way – I think you would have a difficult time arguing that it is highly implausible that factors associated directly with home birth cause worse outcome, so much so that we should be highly suspicious of any apparent correlation, or conclude that there must be some other causal relationship.

  22. IndianaFran – I am not an expert on homebirth, and at present I do not have an independent opinion about the data. But reading over Amy’s post I do not see that she claimed there was a robust consensus based upon systematic reviews.

    The data is clearly imperfect, and will likely improve as more studies are done. I read Amy’s post as presenting her analysis of existing data. You are free to criticize and disagree with her analysis.

  23. Plonit says:

    Also, whether or not the cause is biological or social is irrelevant to the point of plausibility.

    +++++++++

    Whether the cause is biological or social is relevant to the question of how far we can use the data to support the unqualified assertion “Homebirth increases the risk of neonatal death to double or triple the neonatal death rate at hospital birth.”

  24. Zoe237 says:

    The issue of homebirth is clearly one in which specialists in the U.S. are influenced by factors other than science based medicine, particularly the draw of anecdotes over statistics. I don’t believe that any study with matched cohorts has shown homebirth to be unsafe. Comments keep mentioning all of these studies but I haven’t seen them yet. There is certainly no scientific consensus on the matter, at least in the United States. In most of the rest of the world, the consensus is that homebirth is safe and presented as an entirely reasonable option. Tuteur’s entire argument is based on her own unpublished, informal review of the CDC wonder database, and I wonder about her confirmation bias and her inability to articulate the limitations of this database. Furthermore, the social implications are certainly relevant in which at least two well designed studies have shown homebirth to be safe in certain contexts. Thus, her conclusion could just as easily be that homebirth can be made safer by x,y, and z.

    I seriously doubt Tuteur will go because of the high volume of comments and hits her extremist views garner. It would be nice, however, to see at least a more science based researcher or doctor present a balanced viewpoint on obstetrics. It is quite disturbing, though no doubt satisfying to some, to see that the information posted under obstetrics and gynecology is “the tragic toll of homebirth,” “the mother is the factory,” and “what’s the right c-section rate? higher than you think.”

  25. “I don’t believe that any study with matched cohorts has shown homebirth to be unsafe.”

    The Johnson and Daviss BMJ 2005 study, a cohort study, which claims to show that homebirth is as safe as hospital birth, actually shows that homebirth has nearly triple the rate of neonatal mortality.

    The authors have acknowledged on their own website, Understanding Birth Better, that they used the wrong data for the hospital group (out of date studies extending back to 1969) instead of the correct comparison group (low risk hospital births in 2000.

    The authors explain this with a lie. They claim that the hospital data for 2000 was not available in 2004 when they submitted their study for publication. However, the data was published in 2002. Moreover, Johnson and Daviss had it in their hands the entire time. It is the exact same data they used to make the interventions comparison between homebirth and hospital birth in 2000.

    So the best, most comprehensive study comparing homebirth with a CPM to low risk hospital birth shows homebirth to have nearly triple the neonatal mortality rate. This is also was the CDC data shows.

  26. micheleinmichigan says:

    I am reasonably bored with the home vs hospital birth debate. Just not my cup of tea I guess. But, I think Plonit brings up a good point about the intersection of ethics and science that would make a facinating article.

    As I see it, from a layman’s perspective.

    Science is required to inform ethics. Example: when ethics says that we should address malaria in tropical areas to reduce pain and suffering, science can suggest methods for that goal.

    Ethics is required to inform science. Embryonic stem cells debate, some genetics and cloning and ethical research standards are some examples.

    An ethical decision can NOT be reached if it is based on a misunderstanding or misinterpretation of a scientific discovery. Examples, once again cloning and embryonic stem cell/stem cell.
    Therefore educating the press and public on the facts or process of a disputed SBM (or other science) can be very important.

    The study of ethics and philosophy have particular processes such as thought experiments that can help clarify our (the public and individuals) decisions in the public debate on controversial issues. My inclination is that these are under utilized.

  27. Pattoye says:

    @Zoe237:
    I wholeheartedly applaud your efforts to teach children to teach scientifically. It is very encouraging to hear that and I definitely agree that in children of that age, instilling the ability to think critically and apply the scientific method is more important that memorizing facts about science.

    One small issue: I think trust in experts is an unfortunate necessity in the modern world. The sheer amount and bredth of information about any one topic is usually overwhelming. Trying to do a comprehensive review of the literature surrounding a topic that does not fall within your field of expertise is a daunting and often futile task. I do a great deal of combating creationism with other people and despite having a graduate level understanding of molecular biology find myself seeking outside experts because I simply lack the tools to really comprehend the literature surrounding geologic evidence, fossil evidence, astronomic evidence about the age of the earth, etc… I consider myself fairly well versed in natural science and have a solid foundation in many scientific fields other than molecular biology, but it is no substitute for being an expert. I wouldn’t trust my opinion on a topic where the literature isn’t very clear cut (and sometimes even when it is) even when I have all of the evidence in front of me.

  28. IndianaFran says:

    Steven Novella:
    “I read Amy’s post as presenting her analysis of existing data. You are free to criticize and disagree with her analysis.”
    ……………………………………………………………………………

    The heart of my criticism is this: for an analysis which purports to quantify the risk of home vs hospital birth in the US, and makes an informally stated claim of relative risk (“triple the risk”), there has been an astounding lack of genuine quantitative analysis applied to the raw data. The author’s repeated references to this data as “scientific evidence” are misleading, because this implies that some minimum level of scientific rigor has been applied in deriving these results from the raw statistics.

    As to the biological plausibility, a finding of plausibility does not obviate the researcher’s obligation to give consideration to confounders.
    It is also biologically plausible that the higher proportion of older mothers in the home birth group is responsible for some part of the differing results.
    It is also biologically plausible that the higher proportion of high parity (5th or subsequent birth) mothers in the home birth group is responsible for some part of the difference.
    It is also biologically plausible that the disproportionate presence of certain isolated ethnic groups (e.g. Amish) in the homebirth group can account for some part of the difference (due to higher incidences of genetic flaws).
    It is also statistically plausible that the results are affected by geographic skewing (the CNM/hospital group is overrepresented in the northeast and underrepresented in other regions).
    It is also biologically plausible that socioeconomic differences explain some of the difference in results. This factor cannot be directly observed in the raw data, but can be strongly inferred from other reported factors (e.g. education level).

    It is indeed possible that after accounting for all of the above factors, there may still be an incremental degree of risk associated causally with home birth. (Or not.) But it cannot be claimed to be a scientifically-derived result to completely disregard the presence of all of the known and potential confounders, and attribute all of the difference in results entirely to birth location and attendant, i.e. “Homebirth increases the risk of neonatal death to double or triple the neonatal death rate at hospital birth.”

    Steven Novella also says:
    “at present I do not have an independent opinion about the data”.
    Perhaps you have access to an epidemiologist or statistician who can make an evaluation of these claims?

  29. IndianaFran:

    “It is also biologically plausible that the higher proportion of older mothers in the home birth group is responsible for some part of the differing results.”

    But analyzing the data shows that there is no association between neonatal death rate and maternal age for ages 20-44.

    “It is also statistically plausible that the results are affected by geographic skewing.”

    But analyzing the data shows that there is no association between neonatal death rate and geographical region.

    “It is also biologically plausible that socioeconomic differences explain some of the difference in results.”

    But analyzing the data shows that there is no association between neonatal death a maternal education status in this group.

    “It is also biologically plausible that the disproportionate presence of certain isolated ethnic groups (e.g. Amish) in the homebirth group can account for some part of the difference”

    Even if you remove congenital anomalies entirely, the CNM group has a neonatal mortality rate of 0.26/1000 and the homebirth group has a neonatal mortality rate of 0.61/1000.

    Moreover, congenital anomalies are not necessarily incompatible life. Babies born with significant cardiac anomalies can be saved with appropriate resuscitation and surgery. Of course, if such a baby is born at home, the chances of survival drop precipitously.

    There is really no evidence that the CNM hospital group and the homebirth group differ in any way likely to affect neonatal mortality rates. No matter how you analyze the data, the homebirth group always has higher neonatal mortality than the hospital birth group.

  30. pmoran says:

    I hesitate to contribute outside my main areas of interest, but this talk about plausible likely explanations for the figures raises one that worries me.

    Will those who believe that hospitals engage in a lot of dangerous practices, and that home birth is self-evidently safe and “natural”, be inclined to transfer patients at the first sign of trouble? Or will such cases be seen as a threat to cherished beliefs and interpreted as failure?

    If even a few American midwives are reluctant to transfer patients for such reasons then their deaths would help explain the figures.

    This could be part of the reason why homebirth statistics are carefully guarded.

  31. yeahsurewhatever says:

    The simple correlation is not enough to arrive at a conclusion of causation, but multiple correlations all triangulating on the conclusion that smoking causes lung cancer, combined with biological plausibility, does.

    Um. That’s not actually true. No amount of correlation implies causation. Also, correlations cannot “triangulate on a conclusion that” anything causes anything. That’s not how it works.

    You’re arguing risk, not causation, and that’s the very language the surgeon general uses. Smoking increases risk of lung disease. Not smoking causes cancer. Carcinogens are not even defined as substances that cause cancer. Ionizing radiation is the only one that has been conclusively shown by a definite physical process to cause cancer. Others merely promote risk, by current understanding.

  32. Zoe237 says:

    “The Johnson and Daviss BMJ 2005 study, a cohort study, which claims to show that homebirth is as safe as hospital birth, actually shows that homebirth has nearly triple the rate of neonatal mortality.”

    Yes, you keep repeating that. We get that you think that is what the CDC wonder database shows, but maybe I missed any p values, confidence intervals, control for confounding factors, or evidence that the “out of hospital” option/ “other midwife” option is reliable. I recommend you contact the British Medical Journal to relate these supposed lies, the authors, or publish your own research study to back up your own CDC database analysis. The only evidence you have presented thus far is your own word, while others have presented multiple sources. And these sources are just the recent ones.

    This debate clearly falls under one of the latter two options that Novella details:

    “a genuine controversy with no objective resolution, or simply the conclusion that we currently lack sufficient evidence and do not know the answer.”

    Outcomes of planned home births with certified professional midwives
    large prospective study in North America — Johnson and D

    http://www.bmj.com/cgi/content/full/330/7505/1416?ehom

    Outcomes of planned home births versus planned hospital births after
    regulation of midwifery in British Columbia — Janssen et

    http://www.cmaj.ca/cgi/content/abstract/166/3/315?ijkey=9713201a76141225eafaa8f9491274eceffd7fe1&keytype2=tf_ipsecsha

    http://healthfinder.gov/news/newsstory.aspx?docid=630458

    de Jonge A, van der Goes B, Ravelli A, Amelink-Verberg M, Mol B, Nijhuis J, Bennebroek Gravenhorst J, Buitendijk S, Perinatal mortality and morbidity in a nationwide cohort of 529688 low-risk planned home and hospital births BJOG 2009

    http://news.bbc.co.uk/2/hi/health/7998417.stm
    (over 500,000 births in this one)

    CMAJ. 2009 September; 181(6-7): 359–360.
    doi: 10.1503/cmaj.091240.PMCID: PMC2742151

    Copyright © 1995-2009, Canadian Medical Association
    The safety of home birth: Is the evidence good enough?
    Helen McLachlan, PhD and Della Forster, PhD

    http://www.cochrane.org/reviews/en/ab000352.html

    http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2742151/

  33. Zoe237 says:

    “One small issue: I think trust in experts is an unfortunate necessity in the modern world. The sheer amount and bredth of information about any one topic is usually overwhelming. Trying to do a comprehensive review of the literature surrounding a topic that does not fall within your field of expertise is a daunting and often futile task. I do a great deal of combating creationism with other people and despite having a graduate level understanding of molecular biology find myself seeking outside experts because I simply lack the tools to really comprehend the literature surrounding geologic evidence, fossil evidence, astronomic evidence about the age of the earth, etc… I consider myself fairly well versed in natural science and have a solid foundation in many scientific fields other than molecular biology, but it is no substitute for being an expert. I wouldn’t trust my opinion on a topic where the literature isn’t very clear cut (and sometimes even when it is) even when I have all of the evidence in front of me.”

    That is very true- I’m not saying don’t ever trust experts. But I wouldn’t trust one or even a couple. It’s also what causes many to throw up their hands and give into faith or philosophy because it’s just too darn difficult to sort out scientifically. Strictly speaking, even scientists have little knowledge outside their very limited field of expertise.

    However, there is a big difference between trusting your doctor solely, getting a second opinion, reading what the various medical agencies and organizations have to say, and reading the body of scientific literature, as well as having an understanding of how science works. In other words, don’t trust one expert, but when most of them are saying the same thing, it is a consideration. Many doctors don’t even follow the guidelines of their own trade organizations. All of those are possible for a layperson, given a high school and college general education in statistics, the scientific method, media literacy, and logic.

    If I believed that doctors and scientists had sole access to a priviliged source of information, I might as well not teach. Every liberal arts education has at its core a respect and guidance towards critical thinking, not just basic facts and comprehension. Every science education class I’ve ever had taught that the scientific method is a process that can be relevant to our every day lives. Children and adult students need to be taught to analyze and evaluate varying sources of information.

    From what I’ve read in years past, there seems to be a general lambasting of the liberal tradition of “questioning authority.” It’s an US v THEM mentality in which *some* doctors have a god complex, don’t persuade people particularly well, and take great offense to being questioned. With the advent of the internet, everybody now has access to information that previously was only available in a university library. That can be highly dangerous (and we probably will see vaccination rates continue to decline, until a public health disaster brings them back up). It can also be a fantastic tool for democracy and education. In other words, it’s not the stupid sheep against the guardians of superior knowledge as some science blogs would have us believe. I can’t express this enough: people are really, really turned off by holier than thou scientists and doctors, especially when these people are considered stupid without the MD (as I’ve seen elsewhere). They don’t usually post on science forums, but they are the majority, and I hate to see them abandon scientific thinking because of a few… socially challenged doctors.

    I also see the high specialization of the medical field in this country as perhaps partly a disadvantage. What are these people missing from the big picture when they only know about their own field? The recent mammography guidelines are a case in point- there was no oncologist on the committee that issued them. Is this a strength or weakness? Can a person who treats these diseases day in and out, surrounded by horror stories really be objective? Is the plural of anecdotes data? Or is it beneficial to have people who can take a step back and look at the big picture? Clinician/researchers?

    I would add to the above quote that part of what enables laypeople (or any person) to make judgements about scientific evidence is the ability to recognize when one doesn’t have the expertise or knowledge to make a judgement. Know one’s limits.

    That said, I haven’t been reading about the philosophy of science based medicine for long and am hoping to learn more. Some issues I would like to see addressed in the future (that may have been already, slowly reading archives): reductionist science, organic food and health/nutrition (or debunking therof), the difference between evidence based and science based medicine, whether or not statistics can be used to prove anything, risk assessment, Bayesian models, and science education. And perhaps some practical knowledge- if I get sick and have treatment/drugs/surgery/vaccines offered to me by one or multiple doctors, what kind of objective research can I do as a layperson that goes beyond trusting a website by the mayoclinic(e.g.)?

  34. micheleinmichigan says:

    Zoe237 – all that she said.

    Amen! There are so many things about your comments that I agree with I can’t even list them all.

    You sound like a great teacher! I wish my kids could have you in school.

    As a parent who is in the position of advocating for her child’s medical and educational needs quite often, I am a big proponent of doing internet research before a major doctor, therapy or school visit. This helps me to bring relevant questions to a meeting rather than thinking of all my questions after the meeting. I also check with professional organizations, papers and other parents to see if medical, recommendations seem in line with other doctor’s approaches.

    There is a definite limit to my researching capacity though. Last year I tried researching genetic syndromes. Ha, that way lies madness. The internet is not good for diagnosing any conditions. It can be useful in checking to make sure a treatment or test is standard or reasonable. It can be good to check if a symptom is; put an ice pac on it and see if it gets better, wait til monday and see a doctor or go to ER immediately. Critical thinking skills are imperative for all internet research.

    You are definitely correct that some doctor’s do not even follow guidelines of their own speciality. Which I might accept, if they communicated that and their reasons to the patient, but in my experience they often don’t. I have also had doctor’s prescribe medication and then deny the side effects you report later are related to that medication. The same side effects that the drug company insert states “if you experience these side effect report them to a doctor”.

    So I continue to wonder. Sure this SBM approach sounds great, but how many mainstream doctor’s actually practice it? And how do we patients and patient advocates tell the difference? Is there a SBM seal of approval?

  35. Zoe237 says:

    “So I continue to wonder. Sure this SBM approach sounds great, but how many mainstream doctor’s actually practice it? And how do we patients and patient advocates tell the difference? Is there a SBM seal of approval?”

    Yes, but it’s so much more fun/easier to attack the CAM quacks than look into what your own profession could do better.

    My impression is that doctors prefer their patients not do internet research (or Google U) at all. But there are ways of determining bias even by a non-expert. For example, the vitamin D link on the other autism thread had many problems- non scientific claims, lack of evidence, not being published in a peer reviewed journal.

    So the importance of being able to tease out the plausibility of claims and the biases of experts by laypeople is possible. I went to an anti-vaccine lecture a few months ago because a friend begged me to go and give my input/ ask him questions (she is pg and scared of the H1N1 vaccine).

    http://www.brainguardmd.com/

    He was sponsored by the MI chiropractors association and everybody in the audience was blown away by him by the end, and I was left wondering what planet I lived on that my take could be so different than everybody else’s there. Andrew Moulden is a MD/PhD neuroscientist from Canada who gives anti-vax lectures around the country.

    The first thing he did was ask if there were any MDs in the audience. He then went on for the next two hours showing horrifying pictures of children and adults who had strokes (claiming all of them, or most, were caused by vaccines) and giving a very “scientific” account of the mechanism of vaccine injury (mini strokes, or ischemia). There were appropriate gasps in all the right places and even some audible shocked comments (reminscent of a church). Now, I don’t have the expertise to challenge much of what he said scientifically- I suspect much of it was third year biology. My comment to my friend was that he made all of these plausible scientific arguments, but his conclusion didn’t follow from his premises. He went on and on for two hours about mini strokes and ischemia, but didn’t really say anything about vaccines other than they cause these biological changes. Apparently, nobody else noticed that. He was also selling a product, making outrageous causation statements (vaccines caused ted kennedy’s brain cancer!), and claiming that he has the knowledge to cure autism, but he can’t tell us yet for fear of being prosecuted. I was about the only one out of about 100 people who was… less than convinced. My pregnant friend ultimately chose not to get the vaccine because she was scared. The only way to get through to the majority of people who just don’t think scientifically (besides better public education) is to show pictures of dead pregnant women from H1N1 (anecdotes, the more visual the better). And I can’t support this- it makes science no better than the opposition.

  36. micheleinmichigan says:

    “My impression is that doctors prefer their patients not do internet research (or Google U) at all.”

    Sometimes. I have found my children’s pediatricians (we have a ton of specialists) To be quite tolerant of web research and questions as long as it’s not wacky conspiracy stuff. I think they actually see that my research confirms my trust in them and helps save time.

    I wish my doctor was like that. But, we are working it out.

    “I went to an anti-vaccine lecture a few months ago because a friend begged me to go and give my input/ ask him questions (she is pg and scared of the H1N1 vaccine).”

    You are much more patient than me. I could not sit through such a presentation without laughing openly. The laughing stops the bad feeling in the pit of my stomach. :)

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