Descriptive statistics

Descriptive statistics summarise data before any inference is attempted. Central tendency answers 'what is typical?' — mean (arithmetic average), median (middle value) and mode (most frequent value). Dispersion answers 'how spread out?' — range, variance, standard deviation and coefficient of variation. Worked example: for values 2, 4, 4, 6, 9, the mean is 25/5 = 5, the median is 4, the mode is 4, and the standard deviation is the square root of ((9 + 1 + 1 + 1 + 16)/5) = sqrt(5.6) = 2.37.