Guide · updated 2026-11-04
Outliers: flag, don't delete
How to handle unusual values without changing the population you report on.
Extreme is not wrong
A very large sale or a refund can be real. Deleting it changes totals and hides events that matter.
Flag with a reason
Add a flag column with a declared threshold or rule and let someone with context review. Report how many rows were flagged.
Sentinels are different
Codes like 9999 or -999 that mean no reading are missing values in disguise. Convert them explicitly with a documented rule.
Practise it
- Lesson: Flag outliers, don't delete them
- Exercise: Flag large transactions, keep refunds