What SAS keyword is used to retain the values of variables across iterations of a DATA step?

The keyword used to retain the values of variables across iterations of a DATA step is 'RETAIN.' When you include the RETAIN statement in a DATA step, it ensures that the value of the specified variable remains in memory from one iteration to the next, rather than being reset to missing or its initial value for each iteration. This is particularly useful for cumulative calculations or for maintaining state over multiple rows of data.

For example, if you are calculating a running total or carrying over a value from one observation to the next, the RETAIN statement allows you to access the previous value during the current iteration of the loop. By using RETAIN, the program keeps these variables’ values intact until they are explicitly changed in the DATA step.

The other keywords do not serve this purpose: 'KEEP' is used to specify which variables to keep in the output dataset, 'PRESERVE' is not a recognized SAS keyword in this context, and 'HOLD' does not exist in SAS vocabulary related to retaining values in a DATA step. Thus, RETAIN is the appropriate choice for this functionality.