[R] Ancova doesn't return test statistics

[Gustaf Rydevik]

On Nov 15, 2007 4:36 PM, Johan A. Stenberg <johan.stenberg at emg.umu.se> wrote:
> Dear all,
>
> I'm quite sure that this is a stupid question, but I'll ask anyway.
> I want to perform an ANCOVA with two continuous factors and three
> categorical factors.
>
> Plant population growth rate (GR) = dependent variable
> Seed reduction due to herbivory (SR) = continuous explanatory variable
> Herbivore species (HS, 2 levels) = categorical explanatory variable
> Population (Pop, 24 levels) = categorical explanatory variable
> Population size (Popsize) = continuous explanatory variable
> Year (Year, 16 levels) = categorical explanatory variable
>
> My model is technically simple:
>
> model<-aov(GR~SR*HS*Pop*Popsize*Year)
>
> However, R is not returning any F and P values – only Df, Sum Sq and
> Mean Sq. I have to remove either Year or Pop in order to get the test
> statistics. Why is this?
>
> Thank you in advance!
> Johan A. Stenberg, Umea University, Sweden
>

Hi,

How much data do you have? a model on the data with all interactions
included ,as in your example, requires estimating well over 500
parameters. Even the largest data sets might be strained by this.
Supposedly, that's why the aov doesn't give p-values.
(though since you don't give a reproducible example, I can't be certain)

I'd suggest reducing the number of interactions. I'm fairly certain
that you don't want 4- or 5-way interactions for example-they tend to
be hard to interpret.

/Gustaf

-- 
Gustaf Rydevik, M.Sci.
tel: +46(0)703 051 451
address:Essingetorget 40,112 66 Stockholm, SE
skype:gustaf_rydevik