Many problems in Stata call for selection of observations according to true or false conditions, indicator variables flagging the same, groupwise calculations, or a prearranged sort order. The example of finding the first (earliest) and last nonmissing value in panel or longitudinal data is used to explain and explore these devices and how they may be used together. Negating an indicator variable has the special virtue that selected observations may be sorted easily to the top of the dataset.
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