Speaking Stata: Counting groups, especially panels
Nicholas J. Cox
Department of Geography
Durham University
Durham City, UK
[email protected]
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Abstract. 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.
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Nicholas J. Cox
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data management, panels
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