Compute Allowed Changes in Adding to or Dropping from a Formula

Usage

add.scope(terms1, terms2)
drop.scope(terms1, terms2)
factor.scope(factor, scope)

Arguments

terms1 the terms or formula for the base model.
terms2 the terms or formula for the upper (add.scope) or lower (drop.scope) scope. If missing for drop.scope it is taken to be the null formula, so all terms (except any intercept) are candidates to be dropped.
factor the "factor" attribute of the terms of the base object.
scope a list with one or both components drop and add giving the "factor" attribute of the lower and upper scopes respectively.

Description

add.scope and drop.scope compute those terms that can be individually added to or dropped from a model while respecting the hierarchy of terms.

Details

factor.scope is not intended to be called directly by users.

Value

For add.scope and drop.scope a character vector of terms labels. For factor.scope, a list with components drop and add, character vectors of terms labels.

Author(s)

B.D. Ripley

See Also

add1, drop1, aov, lm

Examples

add.scope( ~ a + b + c + a:b,  ~ (a + b + c)^3)
# [1] "a:c" "b:c"
drop.scope( ~ a + b + c + a:b)
# [1] "c"   "a:b"

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