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


From   Ricardo Ovaldia <ovaldia@yahoo.com>
To   statalist@hsphsun2.harvard.edu
Subject   st: Sample size for ttest - (am I wrong?)
Date   Tue, 18 Feb 2003 09:11:18 -0800 (PST)

Dear all,

I feel like such an idiot and I need some help on this
non-Stata problem. I was in a meeting this morning
with a pathologist that is submitting a proposal to
conduct an experiment to compare the concentration of
a biochemical (Cox-2) in two different tissue cell
lines.  He wants to compare mean concentrations so I
suggested a t-test, after a monotone transformation if
needed. He agreed and proceeded to tell me that
because of monetary constrains, he could only have 20
observations per group and wants to know, what is the
minimum difference that he can detect with 80% power?
I told him that I need some idea of the means and SD
of the measurement and he said that he did not know
because this was a new method, 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.  I felt humiliated and
embarrassed. Does anyone know how to do this, or is
this guy wrong? 

Thank you,
Ricardo


=====
Ricardo Ovaldia, MS
Statistician 
Oklahoma City, OK

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