I am pretty alone in the psych world, most of whom are addicted to SPSS <shudders, clutches crucifix and fumbles for garlic and stake>, but a number who use SAS or specialized programs like Mplus, Eqs or Lisrel. In the psychometrics world, R and Matlab are both popular.
I think you can see who used Stata historically from what it does very well:
-Econometricians (panel data, time series, discrete choice)
-Survey researchers (the survey suite)
-Epidemiologists/biostatisticians (survival analysis, GEE, etc.)
Disciplines influenced by those core areas often pick up software from them So Stata is popular in political science because it is heavily influenced by economics. I am betting J. S. Long's influence sold a lot of copies in sociology.
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* For searches and help try:
* http://www.stata.com/help.cgi?search
* http://www.stata.com/support/statalist/faq
* http://www.ats.ucla.edu/stat/stata/
