Variation is the differences between individuals of the same species, caused by genetic and environmental factors. Surveys into variation give data that are continuous, which means to come in a range, ...
Cooke, P. Jones, R M. Mather, K. Bonsall, G W, and Nelder, J A. 1962. Estimating the components of continuous variation. I. Statistical. Heredity, 17, 115–133.