For guidance on when observational studies can be considered to offer strong support for causation, see the old Bradford Hill criteria, or check out GRADE.


Teresa Benson

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From: Evidence based health (EBH) [mailto:[log in to unmask]] On Behalf Of Caroline Boulind
Sent: Friday, January 28, 2011 10:19 AM
To: [log in to unmask]
Subject: Re: Can RCT help establish causation?


As far as I understand it, that is the purpose of randomisation. If it is done correctly, randomisation distributes all varying factors among the test population evenly between the arms of the study. Obviously, one can't identify all factors that might possibly be related to the intervention and outcome of interest to you, so the important thing about randomisation is that both known and unknown 'confounders' are (theoretically) evenly distributed. This means that the only thing changing between the arms of a trial is the intervention of interest, and differences in outcome between the two arms can therefore be interpreted as being a consequence of the intervention of interest.

I suppose the problem is that there is no way of identifying those so called unknown confounders and it is therefore impossible to say definitively that there are no alternative causative variables that are differentially distributed between the arms of the study...


For me the important point is that a well conducted and reported RCT is infinitely closer to being able to identify a causal relationship between exposure and outcome than an observational study. Until a better design is identified EBM is better placed taking its lead from well conducted RCTs than observational studies alone.
Having said this, I think there is a place for observational studies to be considered in conjunction with RCTs, especially in areas of medicine (such as surgery) where RCTs may be more difficult to implement well.




Dr. Caroline Boulind
Clinical Research Fellow
01935 384559e
>>> "Djulbegovic, Benjamin" <[log in to unmask]> 28/01/2011 15:06 >>>

Dear all

I'd like to post this question to the group that I have been thinking about for some time... Is there a scientific method that allows us to LOGICALLY  distinguish the cause-effect from the coincidence? David Hume, one of the most influential philosophers of all times, concluded that there is no such a  method. This was before RCTs were "invented". Many people have made cogent arguments that (a well done) RCT is the ONLY method that can allow us to draw the inferences about causation. Because this is not possible in the observational studies, RCTs are considered (all other things being equal) to provide more credible evidence than non-RCTs. However, some philosophers have challenged this supposedly unique feature of RCT- they claim that RCTs  cannot (on theoretical and logical ground) establish the relationship between the cause and effect any better than non-RCTs. I would appreciate some thoughts from the group:

1. Can RCT distinguish between the cause and effect vs. coincidences? (under which -theoretical- conditions?) 

If the answer is "no", is there any other method that can help establish the cause and effect relationship?

I believe the answer to this question is of profound relevance to EBM.



 Ben Djulbegovic 



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