How should you implement input validation on a selection screen?

Input validation on a selection screen is crucial for ensuring that the data provided by the user meets the required format and constraints before processing. Implementing validation at the event AT SELECTION-SCREEN is ideal because this event is specifically designated for handling user input before any further actions are taken.

By placing your validation logic in this event, you can intercept the input right after the user has made their selections but before the program has moved on to any subsequent logic or processing. This allows for checks against the entered data, and if any validation errors are found, appropriate error messages can be displayed, preventing the user from proceeding with invalid inputs.

In contrast, implementing checks at other events such as AT SELECTION-SCREEN OUTPUT or END-OF-SELECTION would not be appropriate for real-time validation of user inputs. The output event primarily deals with modifying the appearance of the selection screen and does not allow for input validation. The END-OF-SELECTION event occurs after all selections are processed, making it too late for direct validation of user input on the selection screen.