Combined standard deviation merges multiple datasets. Formula: σ_combined = √[(n₁σ₁² + n₂σ₂² + n₁(x̄₁ - x̄_combined)² + n₂(x̄₂ - x̄_combined)²) / (n₁ + n₂)]. Key concepts: accounts for within-group and between-group variation. Common traps: forgetting to include deviations of group means. Exam tips: organize data clearly, calculate group means first. Time-saving: use structured approach with separate steps. Applications: comparing departments, branch performance, multi-plant analysis. Weighted contribution: larger groups influence combined SD more. Properties: combined SD ≥ maximum individual SD (reflects added variability). Interpretation: combined SD indicates overall consistency across groups. Practice: solve with 2-3 groups progressively. Understanding combination essential for organizational analysis.