Dear Listers:
I am not sure my subject title is correct
My question is as following:
Suppose I have data as following:
obs id date vol X
1 1 01 jan 1995 a -
2 1 28 feb 1995 b a
3 1 31 oct 1996 c a
4 1 25 dec 1996 d d
5 1 29 dec 1996 e c+d
6 1 15 nov 1997 f c+d
7 2 01 jan 1994 g -
8 2 10 jan 1994 h g
9 2 25 jan 1994 i g+h
10 2 3 feb 1994 j g+h+i
11 2 28 feb 1994 k g+h+i+j
12 2 01 jan 1995 l g+h+i+j+k
13 2 11 jan 1995 m g+h+i+j+k+l
What I want is a new varialbe, say call X
Where X is the summation of "VOL" for the same ID, but, the summation only
count for one year behind the date of underlying observation.
For example, take a look for observation #5.
This observation has date ="29 Dec 1996" (for ID=1)
The value of this new variable = all "VOL" occured between "29 Dec 1995" -
"28 Dec 1996".
In this case the value is "c+d"
Take another example.
Let's see the observation #10.
This observation has date= "3 Feb 1994"
I want the X to be the summation of all VOL occured between "3 Feb 1993" to
"2 Feb 1994", ===> =g+h+i
Does anyone have suggestion?
Thank you very much
JT
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