gdd {CDM} R Documentation

## Generalized Distance Discriminating Method

### Description

Performs the generalized distance discriminating method (GDD; Sun, Xin, Zhang, & de la Torre, 2013) for dichotomous data which is a method for classifying students into skill profiles based on a preliminary unidimensional calibration.

### Usage

gdd(data, q.matrix, theta, b, a, skillclasses=NULL)


### Arguments

 data Data frame with N \times J item responses q.matrix The Q-matrix theta Estimated person ability b Estimated item intercept from a 2PL model (see Details) a Estimated item slope from a 2PL model (see Details) skillclasses Optional matrix of skill classes used for estimation

### Details

Note that the parameters in the arguments follow the item response model

 logit P( X_{nj}=1 | \theta_n )=b_j + a_j \theta_n

which is employed in the gdm function.

### Value

A list with following entries

 skillclass.est Estimated skill class distmatrix Distances for every person and every skill class skillspace Used skill space for estimation theta Used person parameter estimate

### References

Sun, J., Xin, T., Zhang, S., & de la Torre, J. (2013). A polytomous extension of the generalized distance discriminating method. Applied Psychological Measurement, 37, 503-521.

### Examples

#############################################################################
# EXAMPLE 1: GDD for sim.dina
#############################################################################

data(sim.dina, package="CDM")
data(sim.qmatrix, package="CDM")

data <- sim.dina
q.matrix <- sim.qmatrix

# estimate 1PL (use irtmodel="2PL" for 2PL estimation)
mod <- CDM::gdm( data, irtmodel="1PL", theta.k=seq(-6,6,len=21),
decrease.increments=TRUE, conv=.001, globconv=.001)
# extract item parameters in parametrization b + a*theta
b <- mod$b[,1] a <- mod$a[,,1]
# extract person parameter estimate
theta <- mod$person$EAP.F1

# generalized distance discriminating method
res <- CDM::gdd( data, q.matrix, theta=theta, b=b, a=a )


[Package CDM version 8.2-6 Index]