What does a check digit verify in input validation?

A check digit is a form of redundancy check used for error detection on identifiers, such as barcodes, credit card numbers, or any system where numerical accuracy is crucial. By adding a check digit, a system can verify that the other digits in a sequence are accurate and that the data has not been altered or misentered during input.

When a check digit is calculated, it is derived from the preceding digits in a specific way, and during validation, the system recalculates this check digit to see if it matches the one input by the user. If they match, it indicates that the data entered is likely accurate, thus ensuring the integrity of the numerical data.

In contrast, ensuring data is within a range pertains to boundary checking, confirming correct data formats applies to validation of data structure (like ensuring a date is in the correct format), and checking if data is entered involves determining whether a given field has been populated. While these are all important aspects of input validation, the specific role of the check digit centers around validating the accuracy of the numerical data itself.