Dear Joseph,
Thank you very much!
Kazi
----- Original Message -----
From: "Joseph Coveney" <[email protected]>
To: "Statalist" <[email protected]>
Sent: Thursday, January 06, 2005 4:56 PM
Subject: Re: st: sample size
> Kazi Mizanur Rahman wrote:
>
> The baseline proportion of a an event was 0.9.
> We assume that after providing an intervention in one group for a
particular
> time period we get the proportion as 0.5. We also have a comparison group
> that receives no intervention. After that particular time period the
> proportion becomes 0.8 in the comparison group. How to calculate the
sample
> size with 80% power and alpha .05 ?
>
> --------------------------------------------------------------------------
------
>
> There are undoubtedly some analytical solutions to this power analysis,
but
> have you considered simulation? See A. H. Feiveson, Power by simulation.
> _The Stata Journal_ 2(2):107-24, 2002. ( www.stata-journal.com/sj2-2.html
for
> an abstract.) Something analogous to the do-file below might be worth
> considering. You'd need to take into account any assumptions regarding
> within-subject correlation, if your study design calls for repeated
> measurements.
>
> Joseph Coveney
>
> clear
> set more off
> set seed `=date("2005-01-06", "ymd")'
> *
> program define kazisim, rclass
> replace res0 = uniform() < 0.9
> replace res1 = uniform() < (0.8 - `1' * trt * 0.3)
> logit res1 res0 trt, nolog
> test trt
> return scalar p = r(p)
> end
> * Power (under the alternative hypothesis)
> forvalues samplesize = 50(10)100 {
> display in smcl as result "`samplesize'"
> clear
> quietly set obs `samplesize'
> generate byte trt = _n > _N / 2
> quietly generate byte res0 = .
> quietly generate byte res1 = .
> simulate "kazisim 1" p = r(p), reps(1000)
> generate byte pos = (p < 0.05)
> summarize pos
> }
> * Test size (under the null hypothesis)
> forvalues samplesize = 50(10)100 {
> display in smcl as result "`samplesize'"
> clear
> quietly set obs `samplesize'
> generate byte trt = _n > _N / 2
> quietly generate byte res0 = .
> quietly generate byte res1 = .
> simulate "kazisim 0" p = r(p), reps(1000)
> generate byte pos = (p < 0.05)
> summarize pos
> }
> exit
>
> *
> * For searches and help try:
> * http://www.stata.com/support/faqs/res/findit.html
> * http://www.stata.com/support/statalist/faq
> * http://www.ats.ucla.edu/stat/stata/
*
* For searches and help try:
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