Arithmetic mean (average) x̄ = Σx/n. For grouped data: x̄ = Σ(midpoint × frequency)/n. Properties: (1) Sum of deviations = 0; (2) Minimizes sum of squared deviations; (3) Affected by outliers. Example: Data {2, 4, 6, 8, 10}. Mean = 30/5 = 6. For grouped: Classes [0-5) freq 3, [5-10) freq 7, [10-15) freq 5. Midpoints 2.5, 7.5, 12.5. Mean = (2.5×3 + 7.5×7 + 12.5×5)/(3+7+5) = 115/15 ≈ 7.67. Advantages: Uses all data; familiar. Disadvantages: Affected by outliers. Exam tip: Calculate carefully for grouped data. Understand mean represents average value. Compare with median for skewed data.