Measures of dispersion (spread/variability) indicate data consistency. Range: max - min, simplest measure but sensitive to outliers. Variance (σ²): average squared deviation from mean. Formula: σ² = Σ(x - x̄)² / n. Standard Deviation (σ): square root of variance. Key concepts: larger dispersion means less consistent data. Common traps: confusing sample vs population formulas, squaring errors. Exam tips: use calculator carefully for variance calculation. Time-saving: coefficient of variation (CV = σ/x̄) × 100 compares relative spread. Quartile Deviation (QD): (Q₃ - Q₁)/2, robust to outliers. Applications: quality control, risk assessment, performance evaluation. Interpretation: σ indicates data clustering around mean. Chebyshev's inequality: bounds for data within k standard deviations. Understanding dispersion essential for data analysis. Practice with grouped and ungrouped data.