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st: single sample pre/post comparison of proportions

From   "Michael I. Lichter" <MLichter@Buffalo.EDU>
Subject   st: single sample pre/post comparison of proportions
Date   Wed, 10 Jun 2009 22:41:04 -0400

Greetings! I was asked this afternoon about the most appropriate way to test of significance for a comparison of proportions before and after an intervention within a single population, given that the underlying dichotomous variable can only change from NO to YES and not from YES to NO (so that the proportion can only increase from pre to post). I wasn't sure about the answer ...

Is a two-sample test of proportions as in -prtest- reasonable under these circumstances, given a reasonable N? (The actual N=270, which should be OK for -prtest-'s asymptotic statistics, I think.) I think McNemar's test and Cochran's test are inappropriate (like the chi-square test) because they are ultimately tabular and there cell where pre=YES and post=NO is empty.

The specifics are that a group of physicians was presented with an educational intervention intended to prompt adoption of a set of behaviors. Some physicians had already adopted those behaviors, so their pre status (ADOPT = YES) did not change after the intervention; only physicians who had not adopted at the pre-test could change adoption status after the intervention.

There must be some fairly standard way of handling this in epidemiology where for some conditions people can go from disease-free to diseased, but not from diseased to disease-free.

Thanks for any suggestions.

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