Paul,

Try specifying the LAGs first then set them to missing for first.site_id. I think this works becuase the lags are performed on every record.

LAG_STOP = LAG(STOP_DATE); LAG_MIN_STOP = LAG(MIN_STOP); LAG_MAX_STOP = LAG(MAX_STOP);

if first.site_id then do; lag_stop = .; lag_min_stop = .; lag_max_stop = .; end;

-Brandon

On Tue, 6 Mar 2007 13:53:23 -0500, Paul Miller <pmiller@OHTN.ON.CA> wrote:

>Hi Jack, > >SITE_ID is my id variable. And my data are sorted by SITE_ID. I've tried >syntax like what you recommend but it's not working. I understand that >lag retains the value that it had the last time it executed. So >presumably it will retain this value even if I set the lag variables = . >for the first site_id? > >In other words, won't it say "Sure, you set these values to missing. But >I still have the value from the last time I executed. And the next time >I execute I'm going to write those values, even if they're from a >previous site_id"? > >At least this is what seems to be happening. > >Thanks, > >Paul > >Paul J. Miller, Ph.D. >Research Scientist and Statistician >Ontario HIV Treatment Network >1300 Yonge St., Suite 308 >Toronto, Ontario M4T 1X3 >Phone: (416) 642-6486 ext 232 >Fax: (416) 640-4245 > >-----Original Message----- >From: Jack Clark [mailto:JClark@chpdm.umbc.edu] >Sent: March 6, 2007 1:46 PM >To: Paul Miller; SAS-L@LISTSERV.UGA.EDU >Subject: RE: Problem combining LAG with a BY variable > >Paul, > >You mention patient ID in your post, but I only see SITE_ID in your >code. >Is SITE_ID the variable for patient ID? > >If so... > >You could initialize the 3 lag variables to missing each time you start >a >new value of SITE_ID... > > if first.site_id then do; > lag_stop = .; > lag_min_stop = .; > lag_max_stop = .; > end; > > else do; > LAG_STOP = LAG(STOP_DATE); > LAG_MIN_STOP = LAG(MIN_STOP); > LAG_MAX_STOP = LAG(MAX_STOP); > end; > >Keep in mind that the use of first.site_id relies on the input data set >being sorted by SITE_ID and the dataset being SET BY SITE_ID (as it was >in >your code). > > >Jack Clark >Research Analyst >Center for Health Program Development and Management >University of Maryland, Baltimore County > > >-----Original Message----- >From: SAS(r) Discussion [mailto:SAS-L@LISTSERV.UGA.EDU] On Behalf Of >Paul >Miller >Sent: Tuesday, March 06, 2007 1:15 PM >To: SAS-L@LISTSERV.UGA.EDU >Subject: Problem combining LAG with a BY variable > >Hi Everyone, > > > >I'm having what I suspect will be an easy problem for many. I want to >calculate a set of lag variables within each value of a patient id. My >syntax appears below. I'm having trouble with the three lag variables >LAG_STOP, LAG_MIN_STOP, LAG_MAX_STOP. Currently, the lag function is >carrying values from one id forward into the next instead of doing the >lag within each value of id. What's the best way to fix this? > > > >Thanks, > > > >Paul > > > >DATA HAART_INTERRUPT(DROP=EVENT_ID DURATION--HAART); > > RETAIN SITE_ID START_DATE STOP_DATE > > MIN_START MAX_START MID_START MIN_STOP >MAX_STOP MID_STOP; > > SET REGIMENS (WHERE=(HAART=1)); > > BY SITE_ID; > > FORMAT START_DATE--MID_STOP MMDDYY8.; > > MID_START = SUM(MIN_START,MAX_START)/2; > > MID_STOP = SUM(MIN_STOP,MAX_STOP)/2; > > > > LAG_STOP = LAG(STOP_DATE); > > LAG_MIN_STOP = LAG(MIN_STOP); > > LAG_MAX_STOP = LAG(MAX_STOP); > > > > IF START_DATE NE . AND LAG_STOP NE . THEN DO; > > STOP_DURATION = START_DATE - >LAG_STOP; > > MIN_STOP_DURATION = STOP_DURATION; > > MAX_STOP_DURATION = STOP_DURATION; > > END; > > > > IF START_DATE NE . AND LAG_STOP = . THEN DO; > > STOP_DURATION = .; > > MIN_STOP_DURATION = START_DATE - >LAG_MAX_STOP; > > MAX_STOP_DURATION = START_DATE - >LAG_MIN_STOP; > > END; > > > > IF START_DATE = . AND LAG_STOP NE . THEN DO; > > STOP_DURATION = .; > > MIN_STOP_DURATION = MIN_START - >LAG_STOP; > > MAX_STOP_DURATION = MAX_START - >LAG_STOP; > > END; > > > > IF START_DATE = . AND LAG_STOP = . THEN DO; > > STOP_DURATION = .; > > MIN_STOP_DURATION = MIN_START - >LAG_MAX_STOP; > > MAX_STOP_DURATION = MAX_START - >LAG_MIN_STOP; > > END; > >RUN;