Counting panels, and more generally groups, is sometimes possible in Stata
through a reduction command (e.g., collapse, contract,
statsby) that produces a smaller dataset or through a tabulation
command. Yet there are also many problems, especially with irregular sets of
observations for varying times, that do not yield easily to this approach. This
column focuses on techniques for answering such questions while maintaining the
same data structure. Especially useful are the Stata commands
by: and egen and indicator
variables constructed for the purpose. With by: we often
exploit the fact that subscripts are defined within group, not within dataset.
egen functions are often used to produce group-level
statistics. Tagging each group just once ensures that summaries, including
counts, are of groups, not individual observations.
