st: Monthly observations to quarterly averages

[David Musselwhite <david.musselwhite@gmail.com>]

Hi Statalisters,

I have a dataset of unemployment rates by state by month and would  
like to convert these into quarterly averages, so I'd like to take the  
mean of every three observations and store it as a separate variable.

AK  99   Oct     0
AK  99   Nov   5
AK  99   Dec   10

would yield a value of 5, tied to that quarter. I'm imagining that  
collapse is the command I want to use, but am having trouble finding  
an example of this kind of process.

Thanks for your help,
David Musselwhite
Cornell University
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