Mean deviation MD = Σ|xi - x̄|/n (deviations from mean) or Σ|xi - median|/n (from median). Median-based more robust. Example: {2, 4, 6, 8}. Mean = 5. |Deviations|: 3, 1, 1, 3. MD = 8/4 = 2. For grouped data: MD = Σ[f × |midpoint - x̄|]/n. Advantages: Considers all values; resistant to outliers. Disadvantages: Absolute values harder to work with; less common than variance. Coefficient of MD = MD/mean for comparison. Solving: Find deviations; take absolute values; average. Use median for robust version. Exam tip: Understand difference from variance (absolute vs squared). Use when outlier resistance important.