genLordChi2 {difR} | R Documentation |
Calculates the generalized Lord's chi-square statistics for DIF detection among multiple groups.
genLordChi2(irtParam, nrFocal)
irtParam |
numeric: the matrix of item parameter estimates. See Details. |
nrFocal |
numeric: the number of focal groups. |
This command computes the generalized Lord's chi-square statistic (Kim, Cohen and Park, 1995), also called the Qj
statistics, in the specific framework of differential item functioning with multiple groups. It forms the basic command
of difGenLord
and is specifically designed for this call.
The irtParam
matrix has a number of rows equal to the number of groups (reference and focal ones) times the number of items J. The first J
rows refer to the item parameter estimates in the reference group, while the next sets of J rows correspond to the same items in each of
the focal groups. The number of columns depends on the selected IRT model: 2 for the 1PL model, 5 for the 2PL model, 6 for the constrained 3PL model
and 9 for the unconstrained 3PL model. The columns of irtParam
have to follow the same structure as the output of
itemParEst
command (the latter can actually be used to create the irtParam
matrix).
In addition, the item parameters of the reference group and the focal groups must be placed on the same scale. This can be done by using itemRescale
command, which performs equal means anchoring between two groups of item estimates (Cook and Eignor, 1991).
The number of focal groups has to be specified with argument nrFocal
.
A vector with the values of the generalized Lord's chi-square DIF statistics.
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/
Cook, L. L. and Eignor, D. R. (1991). An NCME instructional module on IRT equating methods. Educational Measurement: Issues and Practice, 10, 37-45.
Kim, S.-H., Cohen, A.S. and Park, T.-H. (1995). Detection of differential item functioning in multiple groups. Journal of Educational Measurement, 32, 261-276.
itemParEst
, itemRescale
, difGenLord
# Loading of the verbal data data(verbal) attach(verbal) # Creating four groups according to gender ("Man" or "Woman") and # trait anger score ("Low" or "High") group<-rep("WomanLow",nrow(verbal)) group[Anger>20 & Gender==0]<-"WomanHigh" group[Anger<=20 & Gender==1]<-"ManLow" group[Anger>20 & Gender==1]<-"ManHigh" # Splitting the data into the four subsets according to "group" data0<-data1<-data2<-data3<-NULL for (i in 1:nrow(verbal)){ if (group[i]=="WomanLow") data0<-rbind(data0,as.numeric(verbal[i,1:24])) if (group[i]=="WomanHigh") data1<-rbind(data1,as.numeric(verbal[i,1:24])) if (group[i]=="ManLow") data2<-rbind(data2,as.numeric(verbal[i,1:24])) if (group[i]=="ManHigh") data3<-rbind(data3,as.numeric(verbal[i,1:24])) } # Estimation of the item parameters (1PL model) m0.1PL<-itemParEst(data0, model="1PL") m1.1PL<-itemParEst(data1, model="1PL") m2.1PL<-itemParEst(data2, model="1PL") m3.1PL<-itemParEst(data3, model="1PL") # merging the item parameters with rescaling irt.scale<-rbind(m0.1PL,itemRescale(m0.1PL,m1.1PL), itemRescale(m0.1PL,m2.1PL),itemRescale(m0.1PL,m3.1PL)) # Generalized Lord's chi-square statistics genLordChi2(irt.scale, nrFocal=3)