Date:         Tue, 8 Apr 2008 22:26:39 -0400
Reply-To:     "Howard Schreier <hs AT dc-sug DOT org>"
              <schreier.junk.mail@GMAIL.COM>
Sender:       "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
From:         "Howard Schreier <hs AT dc-sug DOT org>"
              <schreier.junk.mail@GMAIL.COM>
Subject:      Re: Simple LAG Data Step

On Tue, 8 Apr 2008 16:41:43 -0500, Tom White <tw2@MAIL.COM> wrote:

>Hi everyone,
>
>The following problem looks like a simple LAG issue.
>It doesn't work for me when I do a BY PROV_ID.
>
>I wish to create the DIFF variable as shown below.
>
>Thank you.
>T
>
>
>
>PROV_ID ID           MYDATE
>01      111111111            24Feb2003
>01      111111111            24Feb2003
>01      111111111            24Feb2003
>01      111111111            24Feb2003
>
>02      222222222            24Sep2004
>02      333333333            07Sep2003
>
>03      444444444            10Jul2003
>03      444444444            03May2004
>03      444444444            07Dec2005
>
>
>PROV_ID ID           MYDATE         DIFF
>01      111111111            24Feb2003      .
>01      111111111            24Feb2003      0
>01      111111111            24Feb2003      0
>01      111111111            24Feb2003      0
>
>02      222222222            24Sep2004      . /* or 0 doesn't matter*/
>
>02      333333333            07Sep2003      . /* or 0 doesn't matter*/
>
>03      444444444            10Jul2003      .
>03      444444444            03May2004      N1
>03      444444444            07Dec2005      N2
>
>N1= 03May2004 - 10Jul2003
>N2= 07Dec2005 - 03May2004

Since Tom did not present his code and did not define "doesn't work for me",
I'll just start from scratch.

DIF is actually a bit better than LAG here. My preferred solution is

   data need;
   set have;
   by prov_id id mydate;
   diff = ifn( first.id , . , dif(mydate) );
   run;