Guided Notes
Data Validation
Restricting what can be entered: drop-down lists, number and date rules, input messages, and error alerts.
Why validate data?
Slide 1 / 5A formula is only as good as its inputs. Data validation stops bad data at the door: typos in category names, negative quantities, dates in the wrong year, or text where numbers belong.
Select the input cells and choose Data ▸ Data Validation. On the Settings tab, pick what's allowed: Whole number, Decimal, List, Date, Time, Text length, or Custom (a formula).
Validation checks new entries as they're typed — combine it with clear labels and it makes a shared workbook far more reliable.