Date:         Fri, 22 May 2009 11:01:51 -0400
Reply-To:     Anna Cui <cuis@RPI.EDU>
Sender:       "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
From:         Anna Cui <cuis@RPI.EDU>
Subject:      data manipulation problem

Hi guys. I don't know SAS very well, but I think I have a quite
complicated data manipulation problem. I would really appreciate your help
on this.

The data looks like this:

obs         date          var

1              Aug-98      2

1              Feb-99      4

1              Jan-01      1

1              Mar-02      2

1              Mar-02      3

2              Jun-95      5

2              Nov-96      4

2              Sep-98      3

2              Sep-99      2

2              Oct-01      4

2              May-02      6

Each obs has multiple rows by month. Some months appear in multiple rows,
some months do not appear at all. For each obs each month, I need to
calculate the variance of "var" in the past three years, including the
months that do not appear. For example, for obs1 and Feb 1999, I need to
calculate the variance of "var" for its values between Feb.1996 and
Feb.1999, it is equal to variance of 2 and 4. For obs2, Feb 1999, it is
the variance of 3 and 4. If there is only 1 value or no value in the three
years, the variance would take 0.

I hope I explained it clearly. Thank you very much in advance.

Anna