Hi there, Am new to posting - so let me know if haven't included enough info etc.
I have conducted an ancova with two between subjects (group - 3 levels) and education (2 levels) IV's and one covariate. The DV has been transformed using sq root. When I run the analysis I have an interaction between group and the covariate suggesting a lack of homogeneity of regression slopes. In trying to understand the interaction (which was unexpected) - I ran some diagnostic tests on the covariate which I found to be skewed. I decided to transform the covariate (using sq root in line with the DV) - and this normalised the distribution. When re-running the ancova using both of the transformed variables the interaction disappeared. Is this valid? Is it necessary to transform a covariate if: 1) it is non-normally distributed and/or 2) to be consistent with the DV? I should probably mention that although I have homogeneity of variance the sample sizes are small and unequal (46/15/15). A further point, which may or may not have a bearing on the above, is the issue of independance between the DV and covariate. The DV is a percentage of overlap between words ticked on two lists - say a and b i.e. a/b * 100 - and the covariate is b - as participants were able to tick as many or a few words as they wanted - and so I wanted to control for sheer number. Is this valid? Or should the DV and covariate be completely independant? Any help much appreciated. thanx Emma . . ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at: . http://jse.stat.ncsu.edu/ . =================================================================
