# Re: st: sample size

 From "Kazi Mizanur Rahman" <[email protected]> To <[email protected]> Subject Re: st: sample size Date Mon, 10 Jan 2005 08:29:12 +0600

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
>
> *
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*
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