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Date:         Mon, 28 Sep 1998 15:46:46 -0400
Reply-To:     HERMANS1 <HERMANS1@WESTAT.COM>
Sender:       "SAS(r) Discussion" <SAS-L@UGA.CC.UGA.EDU>
From:         HERMANS1 <HERMANS1@WESTAT.COM>
Subject:      Re[2]: How to choose closest date in longitudinal set?
Comments: To: JOSE FERNANDO DIAZ <jdiaz@welchlink.welch.jhu.edu>
Content-Type: text/plain; charset=US-ASCII

In this context the criterion "closest" as opposed to "latest prior to" generally implies minimum absolute time distance from a point on a time line. Whether that provides a good approximation to a last observation prior to onset depends critically on what you are trying to measure.

Content questions of this type actually do have a lot to do with statistical programming. Many scientific studies require close collaboration between content specialists and programmers.

A correct "closest date" solution may prove more complicated than a "latest date prior to" solution. A content specialist has to determine which to use. Sig

______________________________ Question _________________________________ Subject: Re: How to choose closest date in longitudinal set? Author: JOSE FERNANDO DIAZ <jdiaz@welchlink.welch.jhu.edu> at Internet-E-Mail Date: 9/28/98 3:14 PM

Wait, wait, wait. If you use the difference (fecha-fechax) you will get a date before or after the onset. This may or may not have biological significance. For example, the date of immunization should be earlier than the date of antibody testing. Clinical trials are more or less straightforward. Observational studies are more difficult. Is diarrhea a cause or an effect of malnutrition??? Sorry this doesn't have to do with SAS.

Jose F. Diaz, MD Division of Disease Control Department of International Health The Johns Hopkins School of Public Health Phone (home) : 410 685 2155


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