The STRATA statement identifies a single variable to use as a stratification variable in the analysis. This yields tests similar to those discussed in Mantel and Haenszel (1959) and Hoel and Walburg (1972) for binary data and pooled-means tests for continuous data. For example, when you test for prevalence in a carcinogenicity study, it is common to stratify on intervals of the time of death; the first level of the stratification variable might represent weeks 0
52, the second might represent weeks 5380, and so on. In multicenter clinical studies, each level of the stratification variable might represent a particular center.
The following option is available in the STRATA statement after a slash (/):
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WEIGHT=keyword
specifies the type of strata weighting to use when computing the Freeman-Tukey and t tests. Valid keywords are SAMPLESIZE, HARMONIC, and EQUAL. SAMPLESIZE requests weights proportional to the within-stratum sample sizes, and is the default method even if the WEIGHT= option is not specified. HARMONIC sets up weights equal to the harmonic mean of the nonmissing within-stratum CLASS sizes, and is similar to a Type 2 analysis in PROC GLM. EQUAL specifies equal weights, and is similar to a Type 3 analysis in PROC GLM.
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