Which word rhymes with major?

Standard deviation measures spread of data around mean. Small SD means data clustered near mean; large SD means data spread out. Calculated by: find mean, subtract mean from each value, square differences, find mean of squares, take square root.

Variance is standard deviation squared. Both measure dispersion. SD is in original units (naira, kg); variance is in squared units.

Normal distribution: about 68% of data within 1 SD of mean, 95% within 2 SD, 99.7% within 3 SD. Values beyond 2 SD are unusual; beyond 3 SD are rare outliers.