itemPar2PL {difR}R Documentation

Item parameter estimation for DIF detection using 2PL model

Description

Fits the 2PL model and returns related item parameter estimates.

Usage

 itemPar2PL(data)
 

Arguments

data numeric: the data matrix.

Details

itemPar2PL permits to get item parameter estimates from the 2PL model. The output is ordered such that it can be directly used with the general itemParEst command, as well as the methods of Lord (difLord) and Raju (difRaju) and Generalized Lord's (difGenLord) to detect differential item functioning.

The data is a matrix whose rows correspond to the subjects and columns to the items. Missing values are not allowed.

The 2PL model is fitted using marginal maximum likelihood by means of the functions from the ltm package (Rizopoulos, 2006).

Value

A matrix with one row per item and five columns: the estimates of item discrimination a and difficulty b parameters, the related standard errors se(a) and se(b), and the covariances cov(a,b), in this order.

Author(s)

Sebastien Beland
Centre sur les Applications des Modeles de Reponses aux Items (CAMRI)
Universite du Quebec a Montreal
sebastien.beland.1@hotmail.com
David Magis
Research Group of Quantitative Psychology and Individual Differences
Katholieke Universiteit Leuven
David.Magis@psy.kuleuven.be, http://ppw.kuleuven.be/okp/home/
Gilles Raiche
Centre sur les Applications des Modeles de Reponses aux Items (CAMRI)
Universite du Quebec a Montreal
raiche.gilles@uqam.ca, http://www.er.uqam.ca/nobel/r17165/

References

Rizopoulos, D. (2006). ltm: An R package for latent variable modelling and item response theory analyses. Journal of Statistical Software, 17, 1-25. URL: http://www.jstatsoft.org/v17/i05/

See Also

itemPar1PL, itemPar3PL, itemPar3PLconst, itemParEst, difLord, difRaju, difGenLord

Examples

# Loading of the verbal data
data(verbal)

# Getting item parameter estimates 
itemPar2PL(verbal[,1:24])

[Package difR version 1.1 Index]