ANCOVA's Underlying Assumptions
As indicated earlier, the "standard" assumptions
of ANOVA (e.g., normality, homogeneity of variance, sphericity) underlie
ANCOVA as well. You should upgrade or downgrade your evaluation of a study
depending upon the attention given to these assumptions in the situations
where Ftests are biased when assumptions are violated. To illustrate
this point, consider Excerpt 18.22 [not shown here]. Notice how the researchers
tested the equal variance assumption (in addition to the linearity and
homogeneity of regression slopes assumptions). As a recipient of research
claims, you have a right to expect researchers to be no less concerned
about underlying assumptions than was the research team that conducted
the study from which Excerpt 18.22 was taken. Unfortunately, you are likely
to come across many ANCOVA studies in which there is
absolutely no discussion of linearity, equal regression slopes, or homogeneity
of variance.
(From Chapter 18, p. 544)
