Sample size calculation in Stata

[Sandra Alba]

Hi everyone,

I am trying to do a power analysis in Stata for a prevalence given the precision, but cannot find any information on how to do that. Does anybody have any suggestions?

Many thanks,

Sandra

[BXC (Bendix Carstensen)]

Simulate some data, analyse them to see what the precison is.
Then trial an error will allow you to pick the appropriate sample size.

Analytically, the normal approximation to the log-odds gives a confidence interval for
the prvalence based on x out of n:

p = x/n

ci.: lower: p/[p+(1-p)*exp{1.96/sqrt(n*p(1-p))}]
upper: p/[p+(1-p)/exp{1.96/sqrt(n*p(1-p))}]

which allows you to do the trial and error analytically.

Best regards,
Bendix Carstensen
_______________________________________________

Bendix Carstensen
Senior Statistician
Steno Diabetes Center
Niels Steensens Vej 2-4
DK-2820 Gentofte
Denmark
+45 44 43 87 38 (direct)
+45 30 75 87 38 (mobile)
b...@steno.dk http://www.biostat.ku.dk/~bxc
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