Date:         Mon, 24 Nov 2003 18:14:10 +0000
Reply-To:     John Whittington <John.W@MEDISCIENCE.CO.UK>
Sender:       "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
From:         John Whittington <John.W@MEDISCIENCE.CO.UK>
Subject:      Re: how to use lag
Comments: To: Richard Read Allen <peakstat@WISPERTEL.NET>
Content-Type: text/plain; charset="us-ascii"; format=flowed

Whoops.  I recently wrote:

>Richard, I'm afraid there are all sorts of reasons why this code would not
>accomplish what Helen wants.
>
>Firstly, it does not respect the ID boundaries, as Helen wants - i.e. it
>would attempt to 'carry forward' data from one ID to the next.  Secondly,
>the logic of the code is simply not correct.  As written, the value of x
>will ALWAYS be missing, and will not achieve the desired 'carry
>forward'.  However, it is very easy to achieve Helen's desired
>functionality using RETAIN rather than lag functions, something like:
>
>data two (drop = last_y) ;
>   retain last_y ;
>   set one;
>   by id;
>   if first.id then x=y;
>   else if y=. then x=last_y;
>     else x=y;
>   last_y = y ;
>run ;

I had misread Richard's "if y>." as being "if y=.", hence my statement
about x always having a missing value is incorrect.  However, I still stand
by the rest of what I said, and the suggested code (as above).

Kind Regards,


John

----------------------------------------------------------------
Dr John Whittington,       Voice:    +44 (0) 1296 730225
Mediscience Services       Fax:      +44 (0) 1296 738893
Twyford Manor, Twyford,    E-mail:   John.W@mediscience.co.uk
Buckingham  MK18 4EL, UK             mediscience@compuserve.com
----------------------------------------------------------------