significance for a random effect in Mixed Model ANOVA
In a number of cases I want to use mixed-model ANOVA tests where I am interested in whether both the fixed and random effects (and their interactions) are significant. If I use this example
library(nlme) data(Orthodont) anova(lme(distance ~ age + Sex, data = Orthodont, random = ~ 1))
I get the result
numDF denDF F-value p-value
(Intercept) 1 80 4123.156 <.0001
age 1 80 114.838 <.0001
Sex 1 25 9.292 0.0054
How do I also get a significance value for the random factor (Subject)?
Incidentally, why does it seem that people are not generally interested in
whether the random variables are different from each other? In the case of
the Orthodont data (if there was replication at the Subject level i.e. if
you could clone humans [as you can plants]), would it not be interesting
to know if subjects (nested within sex) are different to each other as
well as
if there is an age effect (so to know if underlying genotype is also an
important factor)?
Thanks
Nat Street
Nathaniel Street Ume? Plant Science Centre Department of Plant Physiology University of Ume? SE-901 87 Ume? SWEDEN email: nathaniel.street at plantphys.umu.se tel: +46-90-786 5477 fax: +46-90-786 6676 www.upsc.se http://www.citeulike.org/user/natstreet