Date:         Mon, 29 Apr 1996 18:14:05 -0700
Reply-To:     Robin Way <robin@BCINC.COM>
Sender:       "SAS(r) Discussion" <SAS-L@UGA.CC.UGA.EDU>
From:         Robin Way <robin@BCINC.COM>
Organization: Barakat & Chamberlin, Inc.
Subject:      Re: Lags with BY variables

Michael Friendly wrote:
>
> I'm trying to generate lags of observations in a dataset with
> a separate lagged series for each value of a BY variable (SEQ).
> I tried this:
>
> data lags;
>    set codegen;
>    by seq;
>    if first.seq then do;
>       lag1 = ' ';
>       lag2 = ' ';
>       end;
>    else do;
>       lag1 = lag(code);
>       lag2 = lag2(code);
>       end;
>
> But this does not work for the second and subsequent value of the
> by variable.
 
(snip)
Michael:
I remember consulting SI tech support about a similar problem. I liked Tim
Berryhill's solution to your problem, because (here's the rationale): lags
should not be invoked conditionally (i.e., should not be part of an IF
statement or IF-based DO loop). The right way to handle them is to execute
the lag statement up front, without any qualifications, (such as, xlag1 =
lag(x);) and then reset the value of the dataset var later based on
conditional logic (much like Tim suggested).
 
Robin Way
Barakat & Chamberlin, Inc.
Portland, Oregon
robin@bcinc.com