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Re: st: Sample size for ttest - (am I wrong?)


From   Ronan Conroy <rconroy@rcsi.ie>
To   "statalist hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   Re: st: Sample size for ttest - (am I wrong?)
Date   Tue, 18 Feb 2003 17:22:45 +0000

on 18/02/2003 5:11 pm, Ricardo Ovaldia at ovaldia@yahoo.com wrote:

> and furthermore I did
> not need this information because his friend (another
> "statistician") told him that with 8 observations per
> group he could detect a difference of 1.5 standard
> deviations with 80% power.

. sampsi 0 1.5, sd(1) n(8)

Estimated power for two-sample comparison of means

Test Ho: m1 = m2, where m1 is the mean in population 1
                    and m2 is the mean in population 2
Assumptions:

         alpha =   0.0500  (two-sided)
            m1 =        0
            m2 =      1.5
           sd1 =        1
           sd2 =        1
sample size n1 =        8
            n2 =        8
         n2/n1 =     1.00

Estimated power:

         power =   0.8508

A different of 1.5 standard deviations is very large indeed. Yes, the power
is 85%, but the Mann Whitney statistic is 86%


. mwstati 1.5 1 1, i
Mann-Whitney statistic for this situation is: 0.8556
 
That is, the probability that an observation in group 2 will be higher than
an observation in group 1 is 0.8556.

Two comments:
1. The minimum clinically significant difference in Cox2 may well be less
than 1.5 SD and
2. An 80% power is an unacceptably high risk of failure. I don't get out of
bed for less than 90%.


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

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