st: How to roll up data with a time window and condition

[Lynda Zhang <lzhang@hmdc.harvard.edu>]

I would like to roll up the past 4 quarters of sales for each company:

Company  Year Quarters  Sales  4_Mo_Sales
ABC      1990  1         10
ABC      1990  2         10
ABC      1990  3         15
ABC      1990  4         10     45
ABC      1991  1         20     55
BBC      1989  1         20    BBC      1989  2         10  BBC      
1989  3         20
BBC      1989  4         10     60
BBC      1990  1         15     55
BBC      1990  3         10     35

The resulting calculation I am looking to get is in 4_Mo_Sales.

Note the following:

1) The first 3 quarters don't have 4 full quarters, so I don't 
calculate the 4_Mo_Sales for that 3 quarters

2) For BBC, quarter 2 in 1990 is missing, so I only sum up 3 quarters 
and got 35 in the past 4 quarters

Thanks so much!

Lynda

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