Wednesday, February 22, 2006

st: Using SAMPSI for repeated observations

This might be a fairly elementary query....

I'm trying to use the SAMPSI function to help me design a survey which will compare baseline and follow-up observations in case and control communities.

The STATA help file for SAMPSI explains how to use the function to compare case and control observations in a cross-sectional survey design (ie with one time point), for either a comparison of means or a comparison of proportions. When using it to design a comparison of proportions, SAMPSI of course doesn't ask for the estimated standard deviation of the variable being observed.

The help function also explains how to use SAMPSI to conduct a comparison of means where there is more than one observation over time.

However, there's a gap - no apparent way of calculating the sample size for a two-group comparison of PROPORTIONS where there are repeated observations in time.

Can anyone suggest anything?

Thanks.

MG

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