psychotools (version 0.2-0)

btReg.fit: Bradley-Terry Model Fitting Function

Description

btReg.fit is a basic fitting function for simple Bradley-Terry models.

Usage

btReg.fit(y, weights = NULL, type = c("loglin", "logit"), ref = NULL,
  undecided = NULL, position = NULL, start = NULL, vcov = TRUE, estfun = TRUE,
  ...)

Arguments

y

paircomp object with the response.

weights

an optional vector of weights, interpreted as case weights (integer only).

type

character. Should an auxiliary log-linear Poisson model or logistic binomial be employed for estimation? The latter is only available if not undecided effects are estimated.

ref

character or numeric. Which object parameter should be the reference category, i.e., constrained to zero?

undecided

logical. Should an undecided parameter be estimated?

position

logical. Should a position effect be estimated?

start

numeric. Starting values when calling glm.fit.

vcov

logical. Should the estimated variance-covariance be included in the fitted model object?

estfun

logical. Should the empirical estimating functions (score/gradient contributions be included in the fitted model object?

...

further arguments passed to functions.

Value

• btReg.fit returns an S3 object of class "btReg", i.e., a list with components as follows.
• coefficients
  estimated parameters on log-scale (without the first parameter which is always constrained to be 0),
• vcov
  covariance matrix of the parameters in the model,
• loglik
  log-likelihood of the fitted model,
• df
  number of estimated parameters,
• estfun
  empirical estimating function (also known as scores or gradient contributions),
• weights
  the weights used (if any),
• n
  number of observations (with non-zero weights),
• type
  character for model type (see above),
• ref
  character for reference category (see above),
• undecided
  logical for estimation of undecided parameter (see above),
• position
  logical for estimation of position effect (see above),
• labels
  character labels of the objects compared.

Details

btReg.fit provides a basic fitting function for Bradley-Terry models, intended as a building block for fitting Bradley-Terry trees and Bradley-Terry mixtures in the psychotree package, respectively. btReg.fit returns an object of class "btReg" for which several basic methods are available, including print, plot, summary, coef, vcov, logLik, and worth.

See Also

PCModel.fit, RSModel.fit, RaschModel.fit

Examples

## data
data("GermanParties2009", package = "psychotools")

## Bradley-Terry model
bt <- btReg.fit(GermanParties2009$preference)
summary(bt)
plot(bt)