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st: Cox reg sample size formula in our book


From   Enzo Coviello <enzo.coviello@tin.it>
To   statalist@hsphsun2.harvard.edu
Subject   st: Cox reg sample size formula in our book
Date   Sat, 14 Mar 2009 09:51:38 +0100

Dear Stata Users,

making use of -stpower- I noted the formula at the bottom of pag 338 of "An intoduction to survival analysis using Stata" 2nd ed., where to compute the required number of events the estimates of sigma_squared (the variance) and of R_squared are at the denominator. In the next page the comment is "The terms ... (above mentioned) ... accounts for the the variability in the estimate of beta ... the larger the variance ..., the larger the required number of events ", as it should be.

The question is :
This conclusion seems inconsistent with the fact that the terms for the variability of the estimate are at denominator where the larger values should correspond to lesser value of the required number of events.
Do I (or does the formula) miss something ?

Thanks

Enzo

--
Enzo Coviello
Unità di Epidemiologia e Statistica ASL Bari
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