topic Re: if first.ID selecting observation from baseline only in SAS Programming

if first.ID selecting observation from baseline only

Jest — Tue, 24 Oct 2017 20:35:58 GMT

Hi,

I have a longitudinal dataset of 800 subjects who got measurements at different time points over time. I want to create a dataset that includes only first measurements (baseline)....The data looks like the below:

ID VISIT feature

1 2 .

1 3 3.77

1 4 8.88

1 5 5.22

1 6 2.21

2 1 .

2 3 .

2 4 6.88

2 5 7.77

2 6 8.54

I am using the following code:

data want;

set have;

by ID VISIT;

if first.ID and first.VISIT;

run;

However, it doesn't seem to work. The data should look like this:

ID VISIT feature

1 3 3.77

2 4 6.88

any advice would be appreciated...

Thanks!

Re: if first.ID selecting observation from baseline only

Reeza — Tue, 24 Oct 2017 20:43:28 GMT

Because you want to ignore missing you should drop them as well.

data want;
set have (where =(not missing(feature)));
by ID VISIT;
if first.ID;

run;

Re: if first.ID selecting observation from baseline only

CurlerBob — Tue, 24 Oct 2017 20:41:22 GMT

You need to eliminate the "and first.VISIT". The sort requires using the VISIT variable to order the records so the first visit is the first record for the ID. But for the actual selection, you only need first.ID.

Re: if first.ID selecting observation from baseline only

ChrisNZ — Wed, 25 Oct 2017 05:35:41 GMT

Also note that

first.VIST

is always true when

first.ID

if first.ID and first.VISIT;

is the same as

if first.ID;

To summarise the suggestions:

data WANT;
  set HAVE;
  where FEATURE;
  by ID VISIT;
  if first.ID ;
run;

Re: if first.ID selecting observation from baseline only

Jest — Sat, 28 Oct 2017 15:57:25 GMT

Thank you Reeza. This was very helpful.

Quick question: Is there also a way to pick only the third or fourth observation, and not just the first?

In other words is there a third.ID command?

Thanks.

Re: if first.ID selecting observation from baseline only

Reeza — Sat, 28 Oct 2017 17:34:44 GMT

No, there’s first and last. But you can use first to set a counter and use the counter as desired.

If first.id then counter =1;

else counter+1;