What must the dataset be before using the BY statement with the SORT procedure?

For the SORT procedure in SAS, when using a BY statement, the dataset must already be sorted on the BY variable. This ensures that the data is organized in the correct order for subsequent processing, such as in the use of procedures that require groups of data to be processed individually (e.g., summary statistics).

When a dataset is sorted by the BY variable, it allows SAS to correctly identify and process each group of observations that share the same value for that BY variable. If the dataset were not sorted on the BY variable, the subsequent analysis might yield incorrect results because the observations within the groups would not be contiguous and would be mixed with those from other groups.

The other options do not align with the requirements of the BY statement with the SORT procedure. While a specific format, filtering for missing values, or formatting for output can be relevant in different contexts within SAS, they do not pertain to the necessity of sorting the dataset by the designated BY variable before applying the SORT procedure.