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Re: st: general statistical reasoning question in biomedicalstatistics (no Stata content)


From   Ronan M Conroy <rconroy@rcsi.ie>
To   "statalist hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   Re: st: general statistical reasoning question in biomedicalstatistics (no Stata content)
Date   Tue, 16 Dec 2003 11:11:35 +0000

on 11/12/2003 17:11, Christopher W. Ryan at cryan@binghamton.edu wrote:

> 
> There are two premises in a randomized controlled trial with two arms:
> 
> 1.  The two samples are drawn randomly from the same population
> 2.  The active drug actually has no effect (the null hypothesis)

Two small points. The first is that your first premise is untrue. Clinical
trials are not done by randomly sampling a population, but by randomly
assigning members of a sample (non-random) to two groups. The only way in
which this can be construed as random sampling is if you treat the patients
in the trial as a finite population, in which case, yes, the randomization
ensures that all samples of N/2 are equally probable.

This point is carefully ignored in discussing significance testing in
clinical trials. In practice, it doesn't seem to matter.

The other point is that your second statement is now kind of 'old science'
(Popper vintage). What we are more concerned with is not formulating a null
hypothesis as estimating some effect size and its associated confidence
interval. P-values in themselves do not tell you
- if the effect size was important in real life terms or not
- how precisely the effect was measured.


Ronan M Conroy (rconroy@rcsi.ie)
Lecturer in Biostatistics
Royal College of Surgeons
Dublin 2, Ireland
+353 1 402 2431 (fax 2764)

--------------------
Ugh - what's that bitter taste in my coffee?
www.maketradefair.org

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