Editing Analysis of variance (section)

===Random-effects models===
{{Main|Random effects model}}
Random-effects model (class II) is used when the treatments are not fixed. This occurs when the various factor levels are sampled from a larger population. Because the levels themselves are [[random variable]]s, some assumptions and the method of contrasting the treatments (a multi-variable generalization of simple differences) differ from the fixed-effects model.<ref>Montgomery (2001, Chapter 12: Experiments with random factors)</ref>