Restrict inputs to a list, number range, or date window—prevents garbage IN before formulas and pivots.
List from range
Data → Data Validation → List → source =$G$2:$G$50 of allowed categories.
Messages
Input message guides users; error alert blocks invalid entries.
Dependents
Cascading lists (City depends on Country) need named ranges or LAMBDA in advanced sheets.
Important interview questions and answers
- Q: Why validate?
A: Stops typo categories breaking VLOOKUP and pivot groupings. - Q: List validation?
A: Dropdown from fixed allowed values.
Self-check
- What does list validation do?
- How do typos break lookups?
Tip: Put allowed lists on a hidden Lists sheet and point validation there.
Interview prep
- List validation?
Dropdown of allowed values.