```Date: Thu, 19 Dec 2002 10:55:54 -0500 Reply-To: Steve Albert Sender: "SAS(r) Discussion" From: Steve Albert Subject: Re: help on power analysis David Cassell made a good point (as always!): you have a problem in sampling theory here. You need to understand the sampling design, and the issues involved. (You also haven't described how your health care workers are to be recruited, which may introduce some biases of its own, or how they're told to select the n individuals from their communities, which may introduce other biases, affect the correlation between observations in each cluster, etc.) I'd agree with David that you should consult a statistician who's used to designing sampling schemes in order to both properly collect and properly analyze your data. To get background to help you talk to that statistician, some background reading in sampling would be helpful. If you don't have any background in sampling, you might look for "A Sampler on Sampling" or check to see if there is a booklet in one of the Sage series. If you do have some, then you might try reading about cluster sampling in Paul Levy and Stanley Lemeshow's "Sampling of Populations: Methods and Applications", third edition, starting with chapter 8; or for a shorter discussion, Zakkula Govindarajulu's "Elements of Sampling Theory", chapter 9. Govindarajulu discusses cluster sampling for proportions (including sample size) on page 185. A more mathematical treatment is in William Cochran's "Sampling Techniques", 3rd edition (a classic on sampling), with specific discussion of cluster sampling for proportions at the ends of chapters 3 and 9 (which includes a brief mention of sample size). To reiterate: all of the above is for *background*; you still need to consult with a statistician to decide an appropriate sampling scheme, sample size, and analysis method. Textbook treatments make assumptions about the way the sample is drawn that may not be valid in your circumstances, and you need a qualified statistician to design something which is appropriate for *your* study, not for a textbook problem set. Steve Albert Director of Biostatistics Spectrum Pharamceutical Research Corp. San Antonio, TX SAlbert at SpectrumCRO dot com ```

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