The Johnson-Neyman procedure as an alternative to ANCOVA

ANCOVA is a widely used statistical procedure that is particularly useful in analyzing data from experimental designs. There are, however, a number of assumptions that must be tested before proceeding with the ANCOVA. Of particular concern is the assumption of homogeneity of regression slopes (HOS). When the HOS assumption has been violated, the researcher needs to look for an alternative approach to the ANCOVA. The Johnson-Neyman procedure (J-N) is presented as such an alternative. Although the calculations for the procedure are somewhat tedious and are not currently a standard feature of statistical software packages, an alternative approach using SAS syntax codes is presented.