How to do a subtraction between certain rows

[linda0910]

Could somebody help me to deal with this problem, thanks so much.

Basicly, I had thousands of PIN, dates, and mark (which I pasted below). I need to subtract dates between first "new start <400" till last "keep start <400". For example, for PIN=100, I subtract last "keep start <400" 1/7/2010 - first "new start <400" 2/25/2009.

Then, I will need another subtraction for the same pin between "keep start <400" 11/24/2015-  "new start <400" 3/2/2011.

PIN	DATE_COLLECTED	mark
100	2/25/2009	new START <400
100	6/8/2009	keep START  <400
100	1/7/2010	keep START  <400
100	3/2/2011	new START  <400
100	4/26/2011	keep START  <400
100	6/29/2011	keep START  <400
100	12/14/2011	keep START  <400
100	6/6/2012	keep START  <400
100	12/12/2012	keep START  <400
100	6/12/2013	keep START  <400
100	12/11/2013	keep START  <400
100	4/18/2014	keep START  <400
100	10/7/2014	keep START  <400
100	12/16/2014	keep START  <400
100	3/13/2015	keep START  <400
100	11/24/2015	keep START  <400

Any suggestion would be appreciated.

[Astounding · Accepted solution]

Try:

data want;

set have;

by PIN mark notsorted;

if first.PIN then baseline = . ;

retain baseline;

if mark = "new start <400" then baseline = date;

if last.mark and mark = "keep START  <400" then difference = date - baseline;

drop baseline;

run;

[linda0910]

Thanks so much. That's exactly what I want.