topic Re: differences between measurement longitudinal data in SAS Programming

differences between measurement longitudinal data

Jest — Wed, 15 Feb 2017 04:46:51 GMT

Hi,

How can I calculate differences between measurements for a variable in a longitudinal dataset. I want to create a change from time 1 variable for VAR. measurement for time 2 - time 1, time 3 - time 1, time 4 - time 1....time 1 is the baseline.

This is how my data looks:

ID time VAR

1 1 84

1 2 80

1 3 75

1 4 85

2 1 86

2 2 80

2 3 77

2 4 74

Thanks

Re: differences between measurement longitudinal data

PGStats — Wed, 15 Feb 2017 05:05:00 GMT

Use a retain statement to remember the baseline:

data want;
set have; by id;
retain baseline;
if first.id then baseline = var;
change = var - baseline;
drop baseline;
run;

proc print; run;

Re: differences between measurement longitudinal data

mkeintz — Wed, 15 Feb 2017 13:52:12 GMT

This is also a good problem to illustrate the benefits of a well-designed self-merge statement:

data want;
  merge have (where=(time=1) rename=(var=baseline))
        have;
  by id;
  change = var - baseline;
  drop baseline;
run;