What is the purpose of the BY statement in SAS procedures?

The BY statement in SAS procedures serves the purpose of processing data in groups based on the values of specified variables. When the BY statement is used, SAS groups the data according to the unique values of the specified variables and processes each group separately. This allows for individual analysis of each subgroup within the dataset, making it a powerful tool for performing operations like summarization, statistical analysis, or generating reports on distinct segments of data.

For instance, if you have a dataset containing sales data for different regions and you want to generate summary statistics for each region, you would include the BY statement followed by the region variable. This tells SAS to compute the statistics separately for each region, resulting in segmented output rather than a single summary for the entire dataset.

The other choices do not accurately capture the function of the BY statement. While sorting data is important, the BY statement does not sort data in ascending order by itself; that would require a separate SORT procedure. Summarizing data for entire datasets pertains to different procedures that do not involve subsetting via the BY statement. Custom output formats relate to how data is displayed, which is unrelated to the grouping capability provided by the BY statement.