Question

An investigator is interested in a factor with a large number of possible levels. Explain how the conclusions being made about the factor differ when that factor is treated a random factor as opposed to a fixed factor.

Answer 1

• The standard methods for analyzing random-effects models assume that the random factor has infinitely many levels, but usually still work well if the total number of levels of the random factor is at least 100 times the number of levels observed in the data. Situations, where the total number of levels of the random factor is less than 100 times the number of levels observed in the data, require special "finite population" methods.
• An interaction term involving both a fixed and a random factor should be considered a random factor.
• A factor that is nested in a random factor should be considered random.