Item response theory based methods are used to compute
linking constants and conduct chain linking of unidimensional
or multidimensional tests for multiple groups under a common
item design. The unidimensional methods include the Mean/Mean,
Mean/Sigma, Haebara, and Stocking-Lord methods for dichotomous
(1PL, 2PL and 3PL) and/or polytomous (graded response, partial
credit/generalized partial credit, nominal, and multiple-choice
model) items. The multidimensional methods include the least
squares method and extensions of the Haebara and Stocking-Lord
method using single or multiple dilation parameters for
multidimensional extensions of all the unidimensional
dichotomous and polytomous item response models. The package
also includes functions for importing item and/or ability
parameters from common IRT software, conducting IRT true score
and observed score equating, and plotting item response
curves/surfaces, vector plots, information plots, and comparison
plots for examining parameter drift.