Calculates DTF index for a set of items with given item parameters of focal and reference groups.

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Description

Calculates DTF index for a set of items with given item parameters of focal and reference groups.

Usage

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Dtf(cdif = NULL, itemParameters = NULL, irtModel = "2pl",
  focalAbilities = NULL, focalDistribution = "norm", subdivisions = 5000,
  logistic = TRUE, focalDistrExtra = list(mean = 0, sd = 1))

Arguments

cdif

A numeric vector of CDIF values for the test items. If NULL it is calculated using itemParameters and the other arguments.

itemParameters

A list containing "focal" and "reference" item parameters. Item parameters are assumed to be on the same scale. Only used if cdif is NULL. Item parameters for each group should me a matrix with nrow equal to the number of items.

irtModel

A string stating the irtModel to be used. Should be one of "1pl", "2pl", "3pl", "grm" or "pcm". Only used if cdif is NULL.

focalAbilities

If NULL, CDIF is calculated by numerical integration of focal distribution. If not NULL, it must be a numerical vector containing the abilities for the individuals in the focal group. Only used if cdif is NULL.

focalDistribution

A string stating the distribution name to be used for integrating. Only used if focalAbilities and cdif are NULL.

focalDistrExtra

Extra parameters for the focal group distribution function if needed.

subdivisions

A numeric value indicating the number of subdivisions for numerical integration. Only used if focalAbilities and cdif are NULL.

logistic

A logical value stating if the IRT model will use the logistic or the normal metric. Defaults to using the logistic metric by fixing the D constant to 1. If FALSE the constant is set to 1.702 so that the normal metric is used. Only used if cdif is NULL.

Value

dtf Numeric vector with the CDIF index value for each item.

Author(s)

Victor H. Cervantes <vcervantes at icfes.gov.co> <vhcervantesb at unal.edu.co>

References

Raju, N. S., van der Linden, W. J., & Fleer, P. F. (1995). An IRT-based internal measure of test bias with applications for differential item functioning. Applied Psychological Measurement, 19, 353–368.

Examples

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# # Not run

# # data(dichotomousItemParameters)
# # threePlCdif <- Cdif(itemParameters = dichotomousItemParameters, irtModel = '3pl',
# #                     focalAbilities = NULL, focalDistribution = "norm",
# #                     subdivisions = 5000, logistic = TRUE)
# # threePlDtf  <- Dtf(cdif = threePlCdif)

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