Currently odds are being offered of 3 to 1 that Roger Federer will win Wimbledon 2011. The fact that at some stage in the future these odds will seem ridculous (for example Federer may be knocked out in an early round or all his rivals may disappear early on) does not make them inappropriate now. Probabilities depend upon the information available. This is understood by everybody...

...except certain critics of randomised clinical trial who insist on claiming that the probabilities calculated in connection with them are irrelevant because with perfect knowledge of all confounding factors we could show that they did not apply. However these probabilities do not require perfect balance of all confounding factors; they only require that the distribution of these confounding factors in probability be known. In fact, one could go further and say that if they were known to be perfectly balanced the standard probability calculation from a clinical trial would be incorrect - you should not calculate probabilities for a matched pair design as if it were completely randomised.

Stephen
Stephen Senn

Professor of Statistics
School of Mathematics and Statistics
Direct line: +44 (0)141 330 5141
Fax: +44 (0)141 330 4814
Private Webpage: http://www.senns.demon.co.uk/home.html

University of Glasgow
15 University Gardens
Glasgow G12 8QW

The University of Glasgow, charity number SC004401
________________________________________
From: Jim Walker [[log in to unmask]]
Sent: 31 January 2011 04:08
To: Stephen Senn; [log in to unmask]
Subject: Re: Can RCT help establish causation? A philosopher's reply

Perhaps thinking of the problem in engineering terms (rather than
philosophical) can be useful:
The goal in machining parts is not perfection, but achieving acceptable
tolerances for the part (based on its intended use).
From this perspective, the relevant question is something like "Is the
likelihood that this observed relationship is due to chance low enough
that it is appropriate to act as if it were a causal relationship?"


Then we can avoid the trap of treating RCTs as if they were
qualitatively different from (rather than quantitatively superior to)
other study designs. That is, a well done non-RCT study may be entirely
adequate to prompt a change in practice (e.g., Semmelweis).

Jim

James M. Walker, MD, FACP
Chief Medical Information Officer
Geisinger Health System

The best way to predict the future is to invent it.
                                               - Alan Kay
>>> Stephen Senn 01/30/11 6:48 PM >>>
However, I don't think that it is necessary for RCTs to control for all
confounders for valid inference to be based on them and it is precisley
this point that I think many commentators are confused on.
Stephen


Stephen Senn

Professor of Statistics
School of Mathematics and Statistics
Direct line: +44 (0)141 330 5141
Fax: +44 (0)141 330 4814
Private Webpage: http://www.senns.demon.co.uk/home.html

University of Glasgow
15 University Gardens
Glasgow G12 8QW

The University of Glasgow, charity number SC004401
________________________________________
From: Evidence based health (EBH) [[log in to unmask]]
On Behalf Of Jeremy Howick [[log in to unmask]]
Sent: 30 January 2011 23:20
To: [log in to unmask]
Subject: Can RCT help establish causation? A philosopher's reply

Dear All,

Stephen is correct that philosophers have been discussing the issue of
how to establish scientific knowledge for centuries * if not longer *
and it is fair to say that there is no widely accepted response to the
problem Hume posed. (Hume famously argued that while we would be 'fools
or madmen' to NOT accept scientific knowledge, inductive reasoning upon
which scientific laws are base have no rational justification (for a
simple explanation see: en.wikipedia.org/wiki/Problem_of_induction).)

However, philosophers (and others) HAVE managed to provide "a scientific
method that allows us to LOGICALLY distinguish coincidence from
causation". See, for example, Cartwright (2007):

journals.cambridge.org/action/displayAbstract?fromPage=online&aid=1002668

A simple way to put Cartwright's claim is: "if RCTs control for all
confounders, then their conclusions about the causal properties of the
intervention are valid".

The problem, of course, is that even the best randomized trial DO NOT
control for ALL confounders. They are better than other methods at
ruling out confounders, but are not perfect. Hence the EBM insistence
that we keep UP TO DATE with the latest evidence that might overturn
previous evidence. I try to explain this clearly in my forthcoming book
(http://www.amazon.co.uk/Philosophy-Evidence-Based-Medicine-Jeremy-Howick/dp/140519667X).

Best wishes,

Jeremy

From: "[log in to unmask]" >
Reply-To: "[log in to unmask]" >
Date: Fri, 28 Jan 2011 15:06:33 +0000
To: "[log in to unmask]" >
Subject: Can RCT help establish causation?

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.

Thanks
 Ben Djulbegovic




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