IF and ELSE commands. IF command subsets the data set by selecting cases satisfying the IF condition. This command should be placed before the procedure in which the subset of the data is to be used. When cases are concatenated, all cases from one file are added to the end of all cases from another file. It can make parallel or nonparallel matches between different files.
Parallel matches combine files sequentially by case.
Nonparallel matches combine files according to the values of one or more BY variables. Both file have the same variables. Cases from the current file will be added to the cases in sample1.sav. They are sample1a.sav sample1b.sav and the current file. Cases are ordered the same in each file.