Further to Donald's comments about investigating the nature of the dependent-covariate interaction is the Johnson-Neyman technique for calculating the region of significance. This will tell you at what cut-points the regression slopes of your treatment groups change from being not significantly different to significant.
This then allows you to put values on the regions that Donald described where group A > group B, group A < group B etc. There is SPSS syntax for the J-P technique available at http://support.spss.com/answernet/details.asp?ID=19193 which in turn was developed from SAS code (reference given). Kylie. Donald Burrill wrote: > If the lines meet, or cross, (or even if they come close to meeting), > then there is a range of values of the covariate for which the groups do > not differ (in the general vicinity of the meeting point; this may be > more complicated to describe, but will be easy enough to see, if you > have more than two groups, since nobody can guarantee that the meeting > point for lines A and B is anywhere near the meeting point for lines B > and C, etc.). > > If the lines do cross (some of them, anyway), then there are probably > regions of the covariate for which > group A > group B, group A < group B, and group A = group B > approximately (in the sense of "no significant difference"). . . ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at: . http://jse.stat.ncsu.edu/ . =================================================================
