Range = Max - Min. Measures spread; simplest dispersion measure. Example: {2, 5, 8, 12} → Range = 12 - 2 = 10. Advantages: Easy to calculate; intuitive. Disadvantages: Only uses extremes; ignores middle values; affected by outliers. Quartile deviation (semi-interquartile range) QD = (Q₃ - Q₁)/2. More stable than range. Coefficient of QD = (Q₃ - Q₁)/(Q₃ + Q₁) for comparison. Example: Q₁ = 3, Q₃ = 11. QD = (11-3)/2 = 4. Solving: Calculate min/max for range. Find quartiles for QD. Advantage: QD robust to outliers. Disadvantage: Ignores 50% of data. Exam tip: Use range for quick spread estimate. Use QD for robust measure.