Hi R Users,

I'm wondering how can I calculate two (or three) way sum of a variable. A
sample data is:

State Month Year Value
NC Jan 1996 1
NC Jan 1996 2
NC Feb 1997 2
NC Feb 1997 3
NC Mar 1998 3
NC Mar 1998 4
NY Jan 1996 4
NY Jan 1996 5
NY Feb 1997 5
NY Feb 1997 6
NY Mar 1998 6
NY Mar 1998 7

I'm trying to sum up "value" column by State*Month and by State*Month*Year.
Also, I may need to calculate mean value along with "sum".

Any help would be greatly appreciated,

Thanks,
Peng

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