The terms fixed and random are commonly used in the regression modeling literature and pertain [Page 509]to whether particular coefficients in a model are treated as fixed or random values. A statistical model is classified as a fixed effects model if all independent variables are regarded as fixed, a random effects model if all independent variables are regarded as random, and a mixed effects model if the independent variables constitute a mix of fixed and random effects. Analytic methods vary depending on the model. The approach selected depends on the nature of the available data and the study objectives.

A fixed variable is one that is assumed to be measured without error. The values of the fixed variable from one study are assumed to be the ...