st: Calculating running cumulative totals

[Adam Chapman <chapman@deakin.edu.au>]

I wonder if anyone can help me with this...

I'm trying to calculate a running cumulative total for observations over 
the previous 12 months (cum12month).  My data looks like:

# Obs  Month  Cum12month
2	1	2
4	2	6
3	3	9
1	4	10
2	5	12
4	6	16
2	7	18
1	8	19
5	9	24
3	10	27
2	11	29
4	12	33
5	13	36
2	14	34
3	15	34
2	16	35
4	17	37
etc

I'm sure this should be simple, but I can't work out an easy way to do this!

Many thanks,

Adam Chapman
Deakin University
Melbourne, VICTORIA

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