Final answer:
It is true that placing a subsetting IF statement early in the DATA step is more efficient because it filters out irrelevant data immediately, saving resources and reducing execution time by avoiding unnecessary computations on data that will be excluded.
Step-by-step explanation:
The statement that the most efficient place to put a subsetting IF statement is as early as possible in the DATA step is generally true. In the context of SAS programming, a DATA step is used to read in and manipulate data. Placing a subsetting IF statement early in the DATA step means that observations that do not meet the condition are dropped before any further processing occurs, which conserves resources and reduces execution time. This efficiency gain happens because SAS does not have to process the entire dataset if certain conditions are not met, thereby avoiding unnecessary computations on data that will ultimately be excluded.
Consider an example where a dataset contains millions of records, but you are only interested in those records where a specific variable, say 'age', is greater than 30. By including if age > 30; at the beginning of the DATA step, SAS immediately dismisses all records that do not fulfill this criterion, only processing the records that are relevant to subsequent steps. If the subsetting IF was placed at the end of the DATA step, SAS would process the entire dataset first, which is less efficient.