APA Dictionary of Psychology

any statistical procedure or experimental design that uses independent variables whose levels are specifically selected by the researcher for study rather than randomly chosen from a wide range of possible values. For example, a researcher may wish to investigate the effects of the available dosages of a certain drug on symptom alleviation. Fixed-effects models generally are intended to make inferences solely about the specific levels of the independent variables actually used in the experiment. Compare mixed-effects model; random-effects model.