ERIC Number: ED409319
Record Type: RIE
Publication Date: 1997-Jan-25
Reference Count: N/A
Testing for Homogeneity of Slopes in Analysis of Covariance: A Tutorial.
Poremba, Kelli D.; Rowell, R. Kevin
Although an analysis of covariance (ANCOVA) allows for the removal of an uncontrolled source of variation that is represented by the covariates, this "correction," which occurs with the dependent variable scores is unfortunately seen by some as a blanket adjustment device that can be used with an inadequate amount of consideration for the homogeneity of slopes assumption. When regression slopes are found not to be parallel, treatment effects will most likely be biased, and there will be a reduction in the efficiency of the analysis. Twenty heuristic data sets coupled with analysis of variance and ANCOVA analyses are provided to illustrate what may occur when the homogeneity of slopes requirement is not met. Even though each of the groups had identical means, variations in the distribution of data for one of the groups studies led to varying slopes. Consequently, three different ANCOVA values resulted, only one of which was accurate. It should be noted that the homogeneity of slopes assumption can be violated to some degree without seriously affecting the robustness of tests of significance in ANCOVA. (Contains 6 tables, 23 figures, and 3 references.) (Author/SLD)
Publication Type: Reports - Evaluative; Speeches/Meeting Papers
Education Level: N/A
Authoring Institution: N/A
Identifiers: Homogeneity of Regression
Note: Paper presented at the Annual Meeting of the Southwest Educational Research Association (Austin, TX, January 25, 1997).