Some categories of data, such as credit limits or the date someone joined a club, have to meet certain criteria. You can make sure data entered into fields meets those criteria (for example, all credit limits are $5,000 or less) by performing data validation. When a user tries to enter data that isn’t appropriate for the field, you can display a message letting the user know what went wrong.
Open a table in Design view.
Click the field for which you want to validate data entry.
Click Validation Rule.