Precautions when using RETAIN

The RETAIN statement allows the value of a data step variable to persist from one observation to the next - without RETAIN, all variables in the Program Data Vector (PDV) are reset to missing before a fresh observation is read from the input dataset. RETAIN is a very useful feature in many cases, but it can also be dangerous.

When processing clinical data, for example, you may well want to retain information from one record for a given patient in order to refer to it while processing a later observation for the same patient. However, in almost all cases you will not want to retain information from one patient when processing a later patient's data - doing so will lead to the first patient's data getting mixed up with another's, and that's not something you want coming up as an audit finding.

As a programmer, you should always be very careful to explicitly clear the values of retained variables between patients, and as a code reviewer, you should be looking for this to have been applied.

Here's a simple example... the following code is intended to carry forward the last non-missing data point for a subject into the single output record:

proc sort data=vitals1; by patient visit_date; run; data vitals2; set vitals1; by patient; retain ret_pulse; * Store any non-missing values; if pulse ne . then ret_pulse = pulse; * Output the last known non-missing value; if last.patient then do; pulse = ret_pulse; output; end; run

proc sort data=vitals1; by patient visit_date; run; data vitals2; set vitals1; by patient; retain ret_pulse; * Clear the retained variable; if first.patient then ret_pulse = .; * Store any non-missing values; if pulse ne . then ret_pulse = pulse; * Output the last known non-missing value; if last.patient then do; pulse = ret_pulse; output; end; run