Decision: what action changes? Unit: what does one row mean? Target: what outcome and horizon? Prediction time: when must inputs exist? Baseline: what simple rule sets minimum value?
2. Audit data
Check label origin, missingness, duplicates, impossible values, time coverage, population coverage, and features unavailable at prediction time.
3. Split first
Match deployment: random for independent stable rows, grouped for repeated entities, temporal for future prediction. Keep test data untouched until choices freeze.