Variance σ² = Σ(xi - x̄)²/n. For sample: s² = Σ(xi - x̄)²/(n-1). Computational formula: σ² = (Σxi²/n) - x̄². Example: {2, 4, 6, 8}. Mean = 5. Deviations: -3, -1, 1, 3. Squared: 9, 1, 1, 9. Variance = 20/4 = 5. Standard deviation σ = √variance. For data: σ = √5 ≈ 2.24. Properties: σ² ≥ 0; σ² = 0 only if all values equal. For grouped data: Use class midpoints. Coefficient of variation CV = (σ/x̄) × 100% for comparing variability across datasets. Exam tip: Population variance divides by n; sample by n-1. Use standard deviation for interpretation (same units as data).