```Date: Fri, 22 May 2009 11:01:51 -0400 Reply-To: Anna Cui Sender: "SAS(r) Discussion" From: Anna Cui 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 ```

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