Variance, or dispersion, roughly refers to the degree of scatter or variability among a collection of observations. For example, in a survey regarding the effectiveness of a political leader, ratings from individuals will differ. In a survey dealing with reading ability among children, the expectation is that children will differ. Even in the physical sciences, measurements might differ from one occasion to the next because of the imprecision of the instruments used. In a very real sense, it is this variance that motivates interest in statistical techniques.

A basic issue that researchers face is deciding how variation should be measured when trying to characterize a population of individuals or things. That is, if [Page 940]all individuals of interest could be measured, how should the variation among ...