Speaking Stata: Distinct observations

Distinct observations are those different with respect to one or more variables, considered either individually or jointly. Distinctness is thus a key aspect of the similarity or difference of observations. It is sometimes confounded with uniqueness. Counting the number of distinct observations may be required at any point from initial data cleaning or checking to subsequent statistical analysis. We review how far existing commands in official Stata offer solutions to this issue, and we show how to answer questions about distinct observations from first principles by using the by prefix and the egen command. The new distinct command is offered as a convenience tool.