How to combine dated data rows in SAS? - sas

Question

I have longitudinal data, but I wish to combine rows if the value of one variable is the same, and update the time variable so that the start and finish time reflects the combined time period. At the end of this only the combined rows and unique rows are kept.
Here is an example
Data have:
Person
Start
Finish
Weight
A
1/1/1988
31/12/1988
78
A
1/1/1989
31/12/1989
78
A
1/1/1990
31/12/1990
78
A
1/1/1991
31/12/1991
81
A
1/1/1992
31/12/1992
82
A
1/1/1993
31/12/1993
82
B
1/1/1968
31/12/1968
56
B
1/1/1969
31/12/1969
55
B
1/1/1970
31/12/1970
55
Data want:
Person
Start
Finish
Weight
A
1/1/1988
31/12/1990
78
A
1/1/1991
31/12/1991
81
A
1/1/1992
31/12/1993
82
B
1/1/1968
31/12/1968
56
B
1/1/1969
31/12/1970
55
What would be the best way of doing this? Thank you all for your time!

Answer 1

The code below will produce your required results from the sample data. It relies on the data being correctly sorted by start and finish.
data want (keep=person start finish weight);
set have (rename= (start=original_start finish=original_finish)); * rename so that the names do not clash with the final variable names;
by person weight notsorted; * assumes that the data are sorted by START and FINISH dates;
retain start; * remembers this variable WHILE we read multiple rows;
format start finish ddmmyy10.;
if first.weight then start=original_start; * record the first START date of each combination of PERSON and WEIGHT;
if last.weight then do;
finish=original_finish; * record the last FINISH date;
output; * only output when we have read the last row for this combination of PERSON and WEIGHT;
end;
run;
If your real data has complications like overlapping time periods then you could use proc summary to get the result:
* use PROC SUMMARY to calculate minimum and maximum for START and FINISH. Only keep the minimum for START and the maximum for FINISH;
proc summary data=have nway;
class person weight;
var start finish;
output out=want2 (drop=_type_ _freq_ min_finish max_start) min=start min_finish max=max_start finish;
run;
or if you need to keep the rows in the same order you can replace class person weight; with by person weight notsorted; but this will cause issues if the rows containing the same person and weight values are not all together in the dataset.
proc summary data=have nway;
by person weight notsorted;
var start finish;
output out=want2 (drop=_type_ _freq_ min_finish max_start) min=start min_finish max=max_start finish;
run;

Answer 2

Suppose the data values for a person contain a time ordered pattern such as
X X X Y X X
and you want 3 rows, 1 for each contiguous period of same valued data values.
You can use DOW processing to compute the start and finish of each contiguous group.
Example:
data have;
input Person $ Start ddmmyy10. Finish ddmmyy10. Weight;
format start finish ddmmyy10.;
datalines;
A 1/1/1988 31/12/1988 78
A 1/1/1989 31/12/1989 78
A 1/1/1990 31/12/1990 78
A 1/1/1991 31/12/1991 81
A 1/1/1992 31/12/1992 82
A 1/1/1993 31/12/1993 82
B 1/1/1968 31/12/1968 56
B 1/1/1969 31/12/1969 55
B 1/1/1970 31/12/1970 55
;
proc sort data=have;
by person start;
data want(keep=person start finish weight);
do until (last.weight);
set have;
by person weight notsorted;
if first.weight then gstart=start;
end;
start = gstart;
run;