D.2 Descriptive Statistics and Correlation

Summary numbers for distributions: mean, median, mode for centre; range, IQR, variance, standard deviation for spread. Effect of constant translation (shifts centre, leaves spread alone) and scaling (centre + spread × |k|, variance × k²) on every statistic. Two-variable analysis: scatter plots, Pearson's r ∈ [−1, +1] for linear association strength + direction (with the famous r ≠ causation warning). Both regression lines: y on x (predicts y) and x on y (predicts x), distinguishing them and warning against extrapolation.