Coefficient of variation CV = (σ/μ) × 100% measures relative variability; compares datasets with different scales/units. CV₁ = 15%, CV₂ = 25% means dataset 2 more variable relative to mean. Combined standard deviation (for k groups): σ²combined = [Σ(ni(σi² + (x̄i - x̄)²))]/n, where x̄ = overall mean. Example: Two groups. Group 1: n₁=20, σ₁=5, x̄₁=50. Group 2: n₂=30, σ₂=8, x̄₂=60. x̄combined = 56. σ²combined = [20(25+(50-56)²) + 30(64+(60-56)²)]/50 = [20(61) + 30(80)]/50 = 3620/50 = 72.4. σcombined ≈ 8.5. Exam tip: Use CV for comparing spread across different datasets. Combined SD accounts for both within-group and between-group variation.