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


From   Ricardo Ovaldia <ovaldia@yahoo.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Sample size for ttest - (am I wrong?)
Date   Tue, 18 Feb 2003 13:44:39 -0800 (PST)

Thank you David and Ronan. I'm confused by the
apparent contradictions in the two posts. I understood
how Ronan calculated the power but its this valid? Or
is the problem (as I originally suspected) that a
"difference of 1.5 SD" doesn't mean anything without
knowing something about the distribution of the two
groups? Am I missing something? 

Thanks in advance, 
Ricardo.

--- David Moore <davem@hartman-group.com> wrote:
> Ricardo,
> 
> You're correct, but the "other" statistician
> probably made a number of
> assumptions to fill in the blanks, so your colleague
> wouldn't be bothered
> with having to think about the substantive issues. 
> I did statistical
> consulting for several years and this (power
> analysis) was one of many
> problems that I had to spend a lot of time
> explaining to clients.  Most of
> the time I ended up walking them through sections of
> Cohen's seminal book
> just to save myself from going insane.  I suspect
> the pathologist's friend
> was using Cohen's (cook)book or a software package
> based on Cohen's formulas
> and suggestions.  If he was a true friend, he would
> have spent the time
> explaining the underlying logic to the pathologist
> and he (the pathologist)
> would have been able to speak intelligently about
> his expectations for the
> means and standard deviations.
> 
> It's been my experience that many experienced and
> fairly competent
> researchers do not really conduct a proper power
> analysis, when it's called
> for.  Your experience with the pathologist is a
> prime example of the
> investigator proceeding without having a full
> understanding of the problem
> at hand.  In my opinion, if he doesn't have a clue
> about the effect size of
> interest, he shouldn't be doing the study.
> 
> Dave

--- Ronan Conroy <rconroy@rcsi.ie> wrote:
> 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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=====
Ricardo Ovaldia, MS
Statistician 
Oklahoma City, OK

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