mlogit (version 0.3-0)

mFormula: Model formula for logit models

Description

Two kinds of variables are used in logit models: alternative specific and individual specific variables. mFormula provides a relevant class to deal with this specificity and suitable methods to extract the elements of the model.

Usage

mFormula(object)
# S3 method for formula
mFormula(object)
is.mFormula(object)
# S3 method for mFormula
model.matrix(object, data, ...)
# S3 method for mFormula
model.frame(formula, data, ..., lhs = NULL, rhs = NULL)

Arguments

object

for the mFormula function, a formula, for the update and model.matrix methods, a mFormula object,

formula

a mFormula object,

data

a data.frame,

lhs

see Formula

rhs

see Formula

...

further arguments.

Value

an object of class mFormula.

Details

Let J being the number of alternatives. The formula may include alternative-specific and individual specific variables. For the latter, J-1 coefficients are estimated for each variable. For the former, only one (generic) coefficient or J different coefficient may be estimated.

A mFormula is a formula for which the right hand side may contain three parts: the first one contains the alternative specific variables with generic coefficient, i.e. a unique coefficient for all the alternatives ; the second one contains the individual specific variables for which one coefficient is estimated for all the alternatives except one of them ; the third one contains the alternative specific variables with alternative specific coefficients. The different parts are separeted by a ``|'' sign. If a standard formula is writen, it is assumed that there are only alternative specific variables with generic coefficients.

The intercept is necessarely alternative specific (a generic intercept is not identified because only utility differences are relevant). Therefore, it deals with the second part of the formula. As it is usual in R, the default behaviour is to include an intercept. A model without an intercept (which is hardly meaningfull) may be specified by includint +0 or -1 in the second rhs part of the formula. +0 or -1 in the first and in the third part of the formula are simply ignored.

Specific methods are provided to build correctly the model matrix and to update the formula. The mFormula function is not intended to be use directly. While using the mlogit function, the first argument is automaticaly coerced to a mFormula object.

Examples

# NOT RUN {
data("Fishing", package = "mlogit")
Fish <- mlogit.data(Fishing, varying = c(2:9), shape = "wide", choice =
"mode")

# a formula with to alternative specific variables (price and catch) and
# an intercept
f1 <- mFormula(mode ~ price + catch)
head(model.matrix(f1, Fish), 2)

# same, with an individual specific variable (income)
f2 <- mFormula(mode ~ price + catch | income)
head(model.matrix(f2, Fish), 2)

# same, without an intercept
f3 <- mFormula(mode ~ price + catch | income + 0)
head(model.matrix(f3, Fish), 2)

# same as f2, but now, coefficients of catch are alternative specific
f4 <- mFormula(mode ~ price | income | catch)
head(model.matrix(f4, Fish), 2)
# }