Sorry, this solution is not working; I used: data aborthist; set aborthist; by descending startdate; if _n_=1 OR (lag(startdate)-startdate ge 180) then output; run; Apparently, the lag function queues the value of the previous observation. However, the previous observation is not necessarily the observation I need for the reference date. The reference date is the date belonging to the previous observation THAT WAS KEPT. For example, I have the dates: May 2014 Dec 2013 Jun 2013 Mar 2013 Jan 2013 Aug 2012 Mar 2012 Jan 2011 Sep 2011 Mar 2011 The following reports SHOULD be extracted: May 2014, Jun 2013, Aug 2012, Dec 2012, Mar 2011. However, only the following reports ARE extracted: May 2014, Jun 2013, March 2011.
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