addterm {MASS}  R Documentation 
Try fitting all models that differ from the current model by adding a single term from those supplied, maintaining marginality.
This function is generic; there exist methods for classes lm
and
glm
and the default method will work for many other classes.
addterm(object, ...) ## Default S3 method: addterm(object, scope, scale = 0, test = c("none", "Chisq"), k = 2, sorted = FALSE, trace = FALSE, ...) ## S3 method for class 'lm': addterm(object, scope, scale = 0, test = c("none", "Chisq", "F"), k = 2, sorted = FALSE, ...) ## S3 method for class 'glm': addterm(object, scope, scale = 0, test = c("none", "Chisq", "F"), k = 2, sorted = FALSE, trace = FALSE, ...)
object 
An object fitted by some modelfitting function. 
scope 
a formula specifying a maximal model which should include the current one. All additional terms in the maximal model with all marginal terms in the original model are tried. 
scale 
used in the definition of the AIC statistic for selecting the models,
currently only for lm , aov and glm models. Specifying scale
asserts that the residual standard error or dispersion is known.

test 
should the results include a test statistic relative to the original
model? The F test is only appropriate for lm and aov models,
and perhaps for some overdispersed glm models. The
Chisq test can be an exact test (lm models with known scale) or a
likelihoodratio test depending on the method.

k 
the multiple of the number of degrees of freedom used for the penalty.
Only k=2 gives the genuine AIC: k = log(n) is sometimes referred
to as BIC or SBC.

sorted 
should the results be sorted on the value of AIC? 
trace 
if TRUE additional information may be given on the fits as they are tried.

... 
arguments passed to or from other methods. 
The definition of AIC is only up to an additive constant: when
appropriate (lm
models with specified scale) the constant is taken
to be that used in Mallows' Cp statistic and the results are labelled
accordingly.
A table of class "anova"
containing at least columns for the change
in degrees of freedom and AIC (or Cp) for the models. Some methods
will give further information, for example sums of squares, deviances,
loglikelihoods and test statistics.
Venables, W. N. and Ripley, B. D. (2002) Modern Applied Statistics with S. Fourth edition. Springer.
quine.hi < aov(log(Days + 2.5) ~ .^4, quine) quine.lo < aov(log(Days+2.5) ~ 1, quine) addterm(quine.lo, quine.hi, test="F") house.glm0 < glm(Freq ~ Infl*Type*Cont + Sat, family=poisson, data=housing) addterm(house.glm0, ~. + Sat:(Infl+Type+Cont), test="Chisq") house.glm1 < update(house.glm0, . ~ . + Sat*(Infl+Type+Cont)) addterm(house.glm1, ~. + Sat:(Infl+Type+Cont)^2, test = "Chisq")