Coefficient of determination R² = r² = proportion of variance in y explained by x. Range: 0 to 1. Example: r = 0.9 → R² = 0.81 (81% of variation explained). Remaining 19% unexplained (residual variance). Interpretation: Higher R² = better fit. R² = 0 (no relationship); R² = 1 (perfect fit). For multiple regression: Adjusted R² accounts for number of variables. Example: Weight vs height. R² = 0.72 means 72% of weight variation due to height; 28% due to other factors. Solving: Square correlation coefficient. Interpret as percentage. Compare models using R². Exam tip: Understand limitations of R² (correlation ≠ causation). Use for model comparison. Practice: Various R² interpretation problems.