Correlation measures how strongly two variables move together. As one goes up, does the other go up too, go down, or wander on its own?
Correlation coefficient r
- +1 — perfect positive (both go up together)
- 0 — no relationship
- −1 — perfect negative (one goes up, the other down)
- Most real data sits somewhere in between
Correlation does NOT imply causation. Ice-cream sales and drowning rates correlate — both rise in summer. Ice cream isn't drowning anyone.