Central tendency, dispersion, skewness, correlation

Statistics summarises data so decisions can be made from it. Central tendency answers 'what is typical?' — the mean, median or mode. Dispersion answers 'how spread out?' — the range, quartile deviation, mean deviation or standard deviation. Worked example: for the values 4, 6, 8, 10, 12, the mean is 8, the median is 8, and the standard deviation is about 2.83 — one number for the centre and one for the spread, and both are needed before any conclusion is drawn.