Appendix A · Ten principles to carry into the job

If you forget everything else, keep these. They are the distilled judgment that took years to internalize, and they apply on your first day and your thousandth.

1. Know the decision before you query. Data serves a choice; find the choice first.
2. Always name the denominator. A rate without its base is a trap.
3. State the grain. Know what one row means before you aggregate.
4. Watch for fan-out. A join that multiplies rows will inflate your totals.
5. Segment before you trust an average. The aggregate often hides the story.
6. Reconcile to a known total. Catch errors before the meeting, not during it.
7. Correlation is a hypothesis, not a finding. Prove cause with an experiment where you can.
8. Pull the minimum personal data. The safest field is the one you never selected.
9. Lead with the answer. Takeaway first, evidence on request.
10. State your caveats. Volunteered uncertainty is what makes your confidence credible.