On Tue, 23 Sep 2003 06:31:36 GMT, Kylie Lange <[EMAIL PROTECTED]> wrote:
> 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. "... at what cut-points ... change from being not significantly different..." Don't you have a linear model? That sounds pretty bogus to me... Off hand, I don't think of good reasons for helping the reader to think that an effect works in *lumps*. I see from google that the technique apparently dates back 50 years, and it is used for showing where the regression lines 'intersect' and are 'not different' in terms of the CI on the intersection. For something that old, it is not well known -- I did not know that name or that it was legitimate until google gave me those hits. >From my brief glance, I don't think it is anything that I will use or recommend. (Am I being unfair? is this important to someone?) The inference drawn from the 95% CI on the intersection-of-regression lines is *cute* but I don't think you can read it that strongly, as a fair point. Also, a point about the technicality: Does the technique get applied *only* in the case of disordinal interactions, or is it also used when the lines do not cross? - I think that one thing that affects me here is that I tend, rather strongly, to regard 'interactions' as being a failure to find the proper elements to model. That is, if the definitions were right, we'd see main effects; while the definitions are wrong, we should be rather calm and quiet about our pronouncements. > > 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). > [ ... ] -- Rich Ulrich, [EMAIL PROTECTED] http://www.pitt.edu/~wpilib/index.html "Taxes are the price we pay for civilization." . . ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at: . http://jse.stat.ncsu.edu/ . =================================================================
