How to obtain previous observation by id

[sasphd]

Hello, 

How to obtain previous observation by id

Have

secid sas_date      TNA

1        201501           345

1        201502         234

2        201501          278

2       201502           876

want 

secid sas_date      TNA

1        201501           .

1        201502         345

2        201501          .

2       201502           278

[ballardw · Accepted solution]

It really is a good idea to be consistent about use of variable names. You said "by id" which makes me guess that you mean secid variable. Having to guess can result in poor performance.

One way: This adds a new variable Ltna so you can compare the results for accuracy.

data want;
   set have;
   by notsorted secid;
   ltna= lag(tna);
   if first.secid then call missing(ltna);
run;

The By statement assumes your data is at least grouped by Secid if not sorted by such. The By statement makes automatic variables that indicate whether the current record is either the first or last of a by group.

The Lag function returns values from the previous record with some limitations. The above approach always gets a previous value and then sets it to missing when it is the first of Secid group.