Data validation: block bad inputs before they hit the cell

Data validation is a spreadsheet gatekeeper: it stops users from typing anything that breaks your rules, like a date in the past, a negative price, or a text where a number is required. The key contrast is that validation rejects the entry at the moment of input, whereas formatting or conditional formatting only changes how the cell looks after the bad data is already there. If you set a cell to accept only whole numbers between 1 and 100, any typed value outside that range is refused immediately.

To spot validation questions in exams, look for phrases like 'input restrictions', 'drop-down lists', or 'error alerts'. A common trick is to confuse validation with data formatting: formatting turns a number red if it is too high, but it does not stop the user from entering it. Another trap is thinking validation can fix existing bad data—it only prevents new bad entries. Always check whether the question asks about preventing input (validation) or highlighting it after the fact (conditional formatting).

A quick memory test: imagine a form that asks for your age. Validation would reject '150' as you type it; conditional formatting would let it in but turn the cell red later. If you remember that validation acts before the data is stored, you will not mix them up. For spreadsheet tasks, always set validation first—it is cheaper than cleaning a mess later.