Denotes the fact that people (and all biological organisms) differ in various ways in which they respond to experimental stimuli, and to a range of factors in the wider environment, exhibiting individual and group differences in their physical and psychological characteristics, aptitudes, and abilities, etc.

The likelihood is that the more variation there is within and across a population the greater will be the sample required (and the more careful must be the selection and analysis of this sample) to accurately gauge the particular aspect(s) of interest about the population, for example, constituents’ inclinations concerning candidates in a forthcoming election or the type of child-care arrangements seen as most beneficial and useful by parents in a particular region (or the country).

(see also: probabilistic, generalizability»generalization, survey)