Correlation measures linear relationship between two variables. Correlation coefficient r: -1 ≤ r ≤ 1. r = 1 (perfect positive), r = -1 (perfect negative), r = 0 (no correlation). Pearson's r = Σ[(xi-x̄)(yi-ȳ)] / √[Σ(xi-x̄)² × Σ(yi-ȳ)²]. Example: Height vs weight. r = 0.85 (strong positive). Interpreting: r > 0.7 strong positive, 0.3-0.7 moderate, < 0.3 weak. Causation: Correlation ≠ causation. Solving: Calculate means. Compute deviations. Apply formula. Exam tip: Distinguish correlation from regression. Use r² (R-squared) = proportion of variance explained. Practice: Scatter plots and correlation calculations.