Summarize, visualize, and understand data before diving into inference.
The mean uses algebraic operations; it's sensitive to outliers. The median is more robust. For symmetric distributions, mean ≈ median ≈ mode.
Variance measures average squared deviation from the mean. Standard deviation has the same units as the data — it's the most widely used spread measure. These connect to the normal distribution via the 68-95-99.7 rule.
Mean = 40/5 = 8
Deviations: −4, −1, 0, 2, 3. Squared: 16, 1, 0, 4, 9
Variance = 30/5 = 6. SD = √6 ≈ 2.45
Skewness describes asymmetry: right-skewed (mean > median, long right tail), left-skewed (mean < median). Kurtosis describes tail heaviness. The normal distribution has skewness 0 and kurtosis 3 (by convention, "excess kurtosis" = 0).