Hi all, I have a question about selecting certain rows of data. The data involve people who have come several times within a period for repeated treatments. Each time they come they were tested several times, ranging from 1 to 10 times. The data is structured as follow: participantID Treatment_Start_Date Assessment_Date Scores 1 13JAN2001 13JAN2001 5 1 13JAN2001 24MAR2001 6 1 13JAN2001 07MAY2001 8 1 15DEC2001 15DEC2001 9 2 01FEB2008 01FEB2008 5 2 01FEB2008 15MAY2008 2 2 01FEB2008 06JAN2009 1 2 15DEC2009 15DEC2009 3 2 15DEC2009 15JAN2010 5 2 26MAY2010 26MAY2010 4 For participant # 1, for example, he received two sets of treatments in different periods: one period starting at 13JAN2001, and the second period starting at 15DEC2001. During the first period, he received assessment concerning the effectivess of treatments on 13Jan2001, 24Mar2001, and 7May2001. The scores of his treatment are 5, 6, and 8 respectively. The second set of treatment is on 15DEC2001, and he was assessed on 15DEC2001. My question is, if I am only interested in the first set of treatment for each participant, is there a way I can retain only the first set of treatment and delete the second, third, fourth sets of treatment (i.e., retaining only 13Jan2001 for the first participant, 01Feb2008 for the second participant, and so on)? Thanks, Chester
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