[R] package MuMI

[Ben Bolker]

Marino Taussig De Bodonia, Agnese <agnese.marino09 <at> imperial.ac.uk> writes:

> I am using the package "MuMI" to run all the possible combinations
>  of variables in my full model, and select
> my best models. When I enter my variables in the original model I 
> write them like this
> 
> lm(y~ a +b +c +a:b)
> 
> However,  "MuMI" will also use the variable b:a, which I do not want 
> in my model.
> 
> How do I stop that from happening?
>

  (1) I think you mean "MuMIn".
  (2) Please send a reproducible example! (Hint: see the posting
guide that is referred to at the bottom of all R-help messages ...)
  (3) [now proceeding to try to guess what you mean ...]

a:b and b:a have identical meanings in the R formula syntax.

  I tried to do something like what I thought you might have meant
and got what seemed to be reasonable answers.

> library(MuMIn)
> dat <- data.frame(y=runif(100),a=factor(sample(1:5,replace=TRUE,size=100)),
+    b= factor(sample(1:5,replace=TRUE,size=100)), c=runif(100))
> m1 <- lm(y~a+b+c+a:b,data=dat)
> dredge(m1)
Global model: lm(formula = y ~ a + b + c + a:b, data = dat)
---
Model selection table 
   (Int)  a b c       a:b k  R.sq    Adj.R.sq  RSS   AIC   AICc  delta   weight
4  0.6385     -0.1643      3 0.03019  0.020290 7.386 29.23 29.48  0.0000 0.420 
1  0.5530                  2 0.00000  0.000000 7.616 30.29 30.42  0.9392 0.262 
7  0.6283   + -0.1461      7 0.09397  0.045770 6.900 30.42 31.64  2.1650 0.142 
3  0.5533   +              6 0.07067  0.031540 7.077 30.96 31.87  2.3890 0.127 
6  0.6134 +   -0.2199      7 0.06700  0.017370 7.105 33.36 34.57  5.0980 0.033 
2  0.5202 +                6 0.01989 -0.021380 7.464 36.28 37.19  7.7100 0.009 
8  0.6232 + + -0.1941     11 0.12050  0.032540 6.698 35.45 38.45  8.9760 0.005 
5  0.5352 + +             10 0.08490  0.004455 6.969 37.42 39.89 10.4100 0.002 
10 0.7433 + + -0.2504 +   27 0.27800  0.034140 5.498 47.71 68.71 39.2400 0.000 
9  0.6193 + +         +   26 0.23150 -0.014420 5.853 51.96 71.19 41.7200 0.000