Compute all the single terms in the
scope argument that can be added to or dropped from the model, fit those models and compute a table of the changes in fit.
add1(object, scope, ...) ## S3 method for class 'default': add1((object, scope, scale = 0, test = c("none", "Chisq"), k = 2, trace = FALSE, ...)) ## S3 method for class 'lm': add1((object, scope, scale = 0, test = c("none", "Chisq", "F"), x = NULL, k = 2, ...)) ## S3 method for class 'glm': add1((object, scope, scale = 0, test = c("none", "Rao", "LRT", "Chisq", "F"), x = NULL, k = 2, ...) drop1(object, scope, ...)) ## S3 method for class 'default': drop1((object, scope, scale = 0, test = c("none", "Chisq"), k = 2, trace = FALSE, ...)) ## S3 method for class 'lm': drop1((object, scope, scale = 0, all.cols = TRUE, test = c("none", "Chisq", "F"), k = 2, ...)) ## S3 method for class 'glm': drop1((object, scope, scale = 0, test = c("none", "Rao", "LRT", "Chisq", "F"), k = 2, ...))
- a fitted model object.
- a formula giving the terms to be considered for adding or dropping.
- an estimate of the residual mean square to be used in computing Cp. Ignored if
- should the results include a test statistic relative to the original model? The F test is only appropriate for
aovmodels or perhaps for
glmfits with estimated dispersion. The Chisq test can be an exact test (
lmmodels with known scale) or a likelihood-ratio test or a test of the reduction in scaled deviance depending on the method. For
glmfits, you can also choose
"Rao"for likelihood ratio tests and Rao's efficient score test. The former is synonymous with
"Chisq"(although both have an asymptotic chi-square distribution).
- the penalty constant in AIC / Cp.
TRUE, print out progress reports.
- a model matrix containing columns for the fitted model and all terms in the upper scope. Useful if
add1is to be called repeatedly. Warning: no checks are done on its validity.
- (Provided for compatibility with S.) Logical to specify whether all columns of the design matrix should be used. If
FALSEthen non-estimable columns are dropped, but the result is not usually statistically meaningful.
- further arguments passed to or from other methods.
drop1 methods, a missing
scope is taken to be all terms in the model. The hierarchy is respected when considering terms to be added or dropped: all main effects contained in a second-order interaction must remain, and so on.
. means ‘what is already there’.
The default output table gives AIC, defined as minus twice log likelihood plus 2p where p is the rank of the model (the number of effective parameters). This is only defined up to an additive constant (like log-likelihoods). For linear Gaussian models with fixed scale, the constant is chosen to give Mallows' Cp, RSS/scale + 2p - n. Where Cp is used, the column is labelled as
Cp rather than
The F tests for the
"glm" methods are based on analysis of deviance tests, so if the dispersion is estimated it is based on the residual deviance, unlike the F tests of
An object of class
"anova" summarizing the differences in fit between the models.
The model fitting must apply the models to the same dataset. Most methods will attempt to use a subset of the data with no missing values for any of the variables if
na.action = na.omit, but this may give biased results. Only use these functions with data containing missing values with great care.
The default methods make calls to the function
nobs to check that the number of observations involved in the fitting process remained unchanged.
Chambers, J. M. (1992) Linear models. Chapter 4 of Statistical Models in S eds J. M. Chambers and T. J. Hastie, Wadsworth & Brooks/Cole.
These are not fully equivalent to the functions in S. There is no
keep argument, and the methods used are not quite so computationally efficient.
Their authors' definitions of Mallows' Cp and Akaike's AIC are used, not those of the authors of the models chapter of S.
require(graphics); require(utils) ## following example(swiss) lm1 <- lm(Fertility ~ ., data = swiss) add1(lm1, ~ I(Education^2) + .^2) drop1(lm1, test = "F") # So called 'type II' anova ## following example(glm) drop1(glm.D93, test = "Chisq") drop1(glm.D93, test = "F") add1(glm.D93, scope = ~outcome*treatment, test = "Rao") ## Pearson Chi-square
Documentation reproduced from R 3.0.1. License: GPL-2.