Question the data before you trust it.
Most data you'll use was collected by someone else, for another purpose. Know its quirks before you make decisions with it.
Four Questions For Any Dataset
Where did it come from?
Which system, collected how, and by whom?
What does each field mean?
“Active” and “customer” mean different things to different teams.
What's missing?
Excluded accounts, failed events, test data mixed in.
What changed over time?
New tracking, new definitions, migrations.
Trust, but verify the pipeline.