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

[log in to unmask] <mailto:[log in to unmask]> 



From: Evidence based health (EBH)
[mailto:[log in to unmask]] On Behalf Of Caroline
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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