[R] prevent aov re-ordering of model terms

[Prof Brian Ripley]

The key is the `keep.order' argument to terms.formula:

> options(contrasts=c("contr.helmert", "contr.poly"))
> library(MASS)
> aov(yield ~ block + N * P + K, npk)
Call:
   aov(formula = yield ~ block + N * P + K, data = npk)

Terms:
                   block        N        P        K      N:P Residuals
Sum of Squares  343.2950 189.2817   8.4017  95.2017  21.2817  218.9033
Deg. of Freedom        5        1        1        1        1        14

Residual standard error: 3.954232
Estimated effects are balanced
> aov(terms(yield ~ block + N * P + K, keep.order=TRUE), npk)
Call:
   aov(formula = terms(yield ~ block + N * P + K, keep.order = TRUE),
    data = npk)

Terms:
                   block        N        P      N:P        K Residuals
Sum of Squares  343.2950 189.2817   8.4017  21.2817  95.2017  218.9033
Deg. of Freedom        5        1        1        1        1        14

Residual standard error: 3.954232
Estimated effects are balanced

Now, I wonder how I knew that ... oh yes, it is in the vr.glm demo.


On Thu, 18 Dec 2003, Pascal A. Niklaus wrote:

> Is there a way to prevent the re-ordering of factors by aov? I do have a 
> three-way interaction that I do want to fit before a two-way interaction 
> (different factors, so they are not nested), but R moves the two-way 
> interaction to the front. I know it generally makes sense to fit the 
> two-way interactions first, but in this case I think I know what I'm doing.
> 
>                    + hv
>                    + hv:rep
>                    + hv:soil
>                    + hv:spdiv
>                    + hv:spdivnom
>                    + hv:fgdiv
>                    + hv:fgdivnom
>                    + hv:soil:spdiv
>                    + hv:soil:spdivnom
>                    + hv:soil:fgdiv
>                    + hv:soil:fgdivnom
>                    + hv:mx                    #<-- that one here
>                    + hv:soil:mx
> 
> mx is a random factor, and I do want to semi-manually test the three-way 
> interactions against "hv:mx". soil is treated as fixed effect.
> 
> I know I can fit the first part, then save the residuals, and then fit 
> the last two terms, but it would be really nice not to have to do this 
> in two steps.


-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595