interval between injections

[Agha]

I should calculate the interval between injections. My data is like this below.

ID	date of injection
A	12/1/2015
A	1/19/2016
A	4/19/2016
A	5/17/2016
B	12/17/2019
B	2/3/2020
B	3/23/2020
B	6/15/2020
C	9/10/2015
C	10/16/2015
C	12/23/2015

The results should be like this: Also should have interval (week)

ID	interval (days)
A	42
A	47
A	90
A	28
B	43
B	45
B	85
C	36
C	42

[PaigeMiller · Accepted solution]

data want;
    set have;
    by id;
    prev_date=lag(date_of_injection);
    if not first.id then do;
        interval=date_of_injection-prev_date;
        output;
    end;
    drop prev_date;
run;

I don't know how you managed to get four intervals for A. When there are four dates for A, there can be only three intervals.

[Agha]

Thanks a lot. It was just an example.

[PaigeMiller]

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@Agha wrote:

Thanks a lot. It was just an example.

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From now on, please give the correct results for your example, as best you can. Making up numbers and making up rows is not a good thing to do.

[SteveDenham]

I hope the results table you want is just an example of the format, as the Interval values do not match the days.  As an example, look at the last 2 values for ID = C.  16 OCT to 23 DEC = remaining days in October + all days in November + days prior to injection in December, which is 15 + 30 +22 =67, not the 42 in the table. 

Other folks following this will be able to tell you how to convert data in date format to SAS date variables, which can be manipulated by standard arithmetic.

SteveDenham

[SteveDenham]

Blatantly stealing @PaigeMiller 's code, here is what I have:

data have;
input id $ date_of_injection $10.;
sasdate=input(date_of_injection,mmddyy10.);       
datalines;
A 12/1/2015
A 1/19/2016
A 4/19/2016
A 5/17/2016
B 12/17/2019
B 2/3/2020
B 3/23/2020
B 6/15/2020
C 9/10/2015
C 10/16/2015
C 12/23/2015
;
run;

data want;
    set have;
    by id;
    prev_date=lag(sasdate);
    if not first.id then do;
        interval=sasdate - prev_date;
        output;
    end;
    drop prev_date;
run;

Which gives output that looks like:

id date_of_injection sasdate interval 
A 1/19/2016          20472   49 
A 4/19/2016          20563   91 
A 5/17/2016          20591   28 
B 2/3/2020           21948   48 
B 3/23/2020          21997   49 
B 6/15/2020          22081   84 
C 10/16/2015         20377   36 
C 12/23/2015         20445   68 

 Interval here is the number of days since the previous injection.  Week can be calculated as week=floor(interval/7), or whatever function you want to get the week.

SteveDenham

[Agha]

I really appreciate