Hello,
I was wondering if anyone could please help me with some simple
questions regarding ANCOVA and the assumption of homogeneity of slopes.
The standard design of ANCOVA assumes the homogeneity of regression
coefficients of the different groups. This assumption can be tested
using the factor ?? covariate interaction, which should subsequently be
removed. However if this assumption is not met the interaction term
shouldn??t be removed, but then the test for group differences only tests
for differences in intercept. This is in many cases not what was
initially intended.
Instead, what one wants is perhaps to determine values of the covariate
at which the groups differ. I??ve seen a description of the
Johnson-Neyman procedure (in Huitema (1980)), which allows to determine
the so called regions of nonsignificance, which sounds a lot like what I
want. The problem is I have very seldom seen it used (at least in my
field of work), but unequal slopes is a common problem. (Searching the
R-help archives, however, didn??t give me a single match.) My first
question is therefore if the Johnson-Neyman procedure is a recommendable
technique. My second question is then of course if somebody knows how to
perform it in R (also for more complex models than a two-groups, one
covariate model).
Hope someone can help me
Wit regards
<>Leif Engqvist
