addtermOld: Try all one-term additions to and deletions from a model
In ordinal: Regression Models for Ordinal Data

Description Usage Arguments Details Value Author(s) See Also Examples

Try fitting all models that differ from the current model by adding or deleting a single term from those supplied while maintaining marginality.

## S3 method for class 'clm2'
addterm(object, scope, scale = 0, test = c("none", "Chisq"),
        k = 2, sorted = FALSE, trace = FALSE,
        which = c("location", "scale"), ...)
## S3 method for class 'clm2'
dropterm(object, scope, scale = 0, test = c("none", "Chisq"),
        k = 2, sorted = FALSE, trace = FALSE,
        which = c("location", "scale"), ...)

`object`	A `clm2` object.
`scope`	for `addterm`: 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. For `dropterm`: a formula giving terms which might be dropped. By default, the model formula. Only terms that can be dropped and maintain marginality are actually tried.
`scale`	used in the definition of the AIC statistic for selecting the models. Specifying `scale` asserts that the dispersion is known.
`test`	should the results include a test statistic relative to the original model? The Chisq test is a likelihood-ratio test.
`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.
`which`	should additions or deletions occur in location or scale models?
`...`	arguments passed to or from other methods.

The definition of AIC is only up to an additive constant because the likelihood function is only defined up to an additive constant.

A table of class "anova" containing columns for the change in degrees of freedom, AIC and the likelihood ratio statistic. If test = "Chisq" a column also contains the p-value from the Chisq test.

Rune Haubo B Christensen

clm2, anova, addterm.default and dropterm.default

options(contrasts = c("contr.treatment", "contr.poly"))

if(require(MASS)) { ## dropterm, addterm, housing
    mB1 <- clm2(SURENESS ~ PROD + GENDER + SOUPTYPE,
                scale = ~ COLD, data = soup, link = "probit",
                Hess = FALSE)
    dropterm(mB1, test = "Chi")       # or
    dropterm(mB1, which = "location", test = "Chi")
    dropterm(mB1, which = "scale", test = "Chi")
    addterm(mB1, scope = ~.^2, test = "Chi", which = "location")
    addterm(mB1, scope = ~ . + GENDER + SOUPTYPE,
            test = "Chi", which = "scale")
    addterm(mB1, scope = ~ . + AGEGROUP + SOUPFREQ,
            test = "Chi", which = "location")

    ## Fit model from polr example:
    fm1 <- clm2(Sat ~ Infl + Type + Cont, weights = Freq, data = housing)
    addterm(fm1, ~ Infl + Type + Cont, test= "Chisq", which = "scale")
    dropterm(fm1, test = "Chisq")
}