Distributions

The shape of the data.

Reading a shape

A histogram shows where values cluster. Symmetric, skewed, bimodal and uniform all mean different things, and all can share the same mean.

Bimodal usually means two groups mixed together that should be separated.
Always plot the data before summarising it.

The normal distribution

Sums of many small independent effects tend towards a bell curve. About 68 percent of values sit within one standard deviation of the mean and 95 percent within two.