Rick, you have it exactly right. I just wanted to expound upon one of your points. Comparing CLASS STATE COUNTY; vs. BY STATE COUNTY; In the output data set using BY, there is one observation for each STATE/COUNTY combination. In the output data set using CLASS, you get those same observations, plus: one observation holding a summary for the entire data set, one set of observations holding a summary for each STATE, and another set of observations holding a summary for each COUNTY. The variable _TYPE_ in the output data sets tells you what the level of summarization is for that observation. The printed reports give you summaries at the most detailed level only, even if the output data sets would be different. And, as Rick noted, the format of the reports would change. Finally, your colleague's recollection is correct. Any observation where a CLASS variable is missing will be thrown out of the analysis. The MISSING option changes that, treating missing values like any other value for a CLASS variable.
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