What determines which statistical software you use? 

 I was recently involved in a discussion among fellow grad students about what determines which statistical software package people use to analyze their data. For example, this recent  market survey  lists 44 products selected from 31 vendors and they do not even include packages like R that many people around Harvard seem to use. Another  survey  conducted by Alan Zaslavsky lists 15 packages while `just’ looking at the available software for the analysis of surveys with complex sample designs. So how do people pick their packages given the plethora of options? Obviously, many factors will go into this decision (departmental teaching, ease of use, type of methods used, etc. etc. etc. ). One particularly interesting factor in our discussion concerned the importance of academic discipline. It seems to be the case that different packages are popular in different disciplines. But how exactly usage patterns vary across fields remains unclear. We wondered whether any systematic data exists on this issue? For example, how many political scientists use R compared to other programs? What about statisticians, economists, sociologists, etc.? Any information would be highly appreciated.