What does the FIRST. variable indicate in a SAS DATA step?

The FIRST. variable is a special automatic variable created by SAS during a DATA step when the BY statement is used. It identifies the first observation in a group of data when the data is sorted by the specified BY variable(s). When processing data, SAS creates a binary flag for each observation within a BY group: if the observation is the first one in that group, the FIRST. variable is set to 1 (or true); otherwise, it is set to 0 (or false). This feature is particularly useful for tasks such as summarizing or performing calculations that should only occur at the beginning of each group, such as initializing variables or generating group-level statistics.

Understanding this concept allows you to effectively manage and manipulate grouped data within a SAS program. The other options mention various aspects of data management, but they do not pertain specifically to what the FIRST. variable signifies in the context of SAS data processing.