psychotree (version 0.11-1)

RaschModel: Model-Generating Function for Rasch Models

Description

RaschModel is a wrapper function that creates a "StatModel" object with certain fitting parameters passed on to RaschModel.fit for fitting simple Rasch models.

Usage

RaschModel(gradtol = 1e-6, deriv = c("sum", "diff", "numeric"))
RaschModel.fit(y, weights = NULL, start = NULL, gradtol = 1e-6,
  deriv = c("sum", "diff", "numeric"), ...)

Arguments

y

object that can be coerced (via as.matrix) to binary 0/1 matrix.

weights

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

start

an optional vector of starting values.

deriv

character. Which type of derivatives should be used for computing gradient and Hessian matrix? Analytical with sum algorithm ("sum"), analytical with difference algorithm ("diff", faster but numerically unstable), or

gradtol, ...

further arguments passed to nlm.

Value

• RaschModel returns an S4 object of class "StatModel" that fits Rasch models with the specified arguments. RaschModel.fit returns an S3 object of class "RaschModel", i.e., a list with components as follows.
• coefficients
  estimated item difficulty parameters (without first item 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,
• data
  the original data supplied (excluding columns without variance),
• weights
  the weights used (if any),
• n
  number of observations (with non-zero weights),
• items
  status indicator (0, 0/1, 1) of all original items,
• na
  logical indicating whether the data contains NAs,
• elementary_symmetric_functions
  List of elementary symmetric functions for estimated parameters (up to order 2; or 1 in case of numeric derivatives),
• nlm_code
  convergence code from nlm,
• iterations
  number of iterations used by nlm,
• gradtol
  tolerance passed to nlm.

Details

RaschModel is the S4 interface for RaschModel.fit so that it can be used in mob. The user does not have to call this directly but can simply use the raschtree interface.

See Also

raschtree