acat {VGAM}R Documentation

Ordinal Regression with Adjacent Categories Probabilities

Description

Fits an adjacent categories regression model to an ordered (preferably) factor response.

Usage

acat(link = "loge", earg = list(),
     parallel = FALSE, reverse = FALSE, zero = NULL)

Arguments

In the following, the response Y is assumed to be a factor with ordered values 1,2,...,M+1, so that M is the number of linear/additive predictors eta_j.

link Link function applied to the ratios of the adjacent categories probabilities. See Links for more choices.
earg List. Extra argument for the link function. See earg in Links for general information.
parallel A logical, or formula specifying which terms have equal/unequal coefficients.
reverse Logical. By default, the linear/additive predictors used are eta_j = log(P[Y=j+1]/P[Y=j]) for j=1,...,M. If reverse is TRUE then eta_j=log(P[Y=j]/P[Y=j+1]) will be used.
zero An integer-valued vector specifying which linear/additive predictors are modelled as intercepts only. The values must be from the set {1,2,...,M}.

Details

By default, the log link is used because the ratio of two probabilities is positive.

Value

An object of class "vglmff" (see vglmff-class). The object is used by modelling functions such as vglm, rrvglm and vgam.

Warning

No check is made to verify that the response is ordinal; see ordered.

Note

The response should be either a matrix of counts (with row sums that are all positive), or a factor. In both cases, the y slot returned by vglm/vgam/rrvglm is the matrix of counts.

For a nominal (unordered) factor response, the multinomial logit model (multinomial) is more appropriate.

Here is an example of the usage of the parallel argument. If there are covariates x1, x2 and x3, then parallel = TRUE ~ x1 + x2 -1 and parallel = FALSE ~ x3 are equivalent. This would constrain the regression coefficients for x1 and x2 to be equal; those of the intercepts and x3 would be different.

Author(s)

Thomas W. Yee

References

Agresti, A. (2002) Categorical Data Analysis, 2nd ed. New York: Wiley.

Simonoff, J. S. (2003) Analyzing Categorical Data, New York: Springer-Verlag.

Documentation accompanying the VGAM package at http://www.stat.auckland.ac.nz/~yee contains further information and examples.

See Also

cumulative, cratio, sratio, multinomial, pneumo.

Examples

data(pneumo)
pneumo = transform(pneumo, let=log(exposure.time))
(fit = vglm(cbind(normal,mild,severe) ~ let, acat, pneumo))
coef(fit, matrix=TRUE)
constraints(fit)
model.matrix(fit)

[Package VGAM version 0.7-7 Index]