LPCM {eRm} R Documentation

## Estimation of linear partial credit models

### Description

This function computes the parameter estimates of a linear partial credit model (LRSM) for polytomuous item responses by using CML estimation.

### Usage

LPCM(X, W , mpoints = 1, groupvec = 1, se = TRUE, sum0 = TRUE,
etaStart)


### Arguments

 X Input data matrix or data frame; rows represent individuals (N in total), columns represent items. Missing values are inserted as NA. W Design matrix for the LPCM. If omitted, the function will compute W automatically. mpoints Number of measurement points. groupvec Vector of length N which determines the group membership of each subject, starting from 1 se If TRUE, the standard errors are computed. sum0 If TRUE, the parameters are normalized to sum-0 by specifying an appropriate W. If FALSE, the first parameter is restricted to 0. etaStart A vector of starting values for the eta parameters can be specified. If missing, the 0-vector is used.

### Details

Through appropriate definition of W the LPCM can be viewed as a more parsimonous PCM, on the one hand, e.g. by imposing some cognitive base operations to solve the items. One the other hand, linear extensions of the Rasch model such as group comparisons and repeated measurement designs can be computed. If more than one measurement point is examined, the item responses for the 2nd, 3rd, etc. measurement point are added column-wise in X.

If W is user-defined, it is nevertheless necessary to specify mpoints and groupvec. It is important that first the time contrasts and then the group contrasts have to be imposed.

Available methods for LPCM-objects are:
print, coef, model.matrix, vcov,summary, logLik, person.parameters.

### Value

Returns on object of class 'eRm' containing:

 loglik Conditional log-likelihood. iter Number of iterations. npar Number of parameters. convergence See code output in nlm. etapar Estimated basic item parameters. se.eta Standard errors of the estimated basic item parameters. betapar Estimated item (easiness) parameters. se.beta Standard errors of item parameters. hessian Hessian matrix if se = TRUE. W Design matrix. X Data matrix. X01 Dichotomized data matrix. groupvec Group membership vector. call The matched call.

### Author(s)

Patrick Mair, Reinhold Hatzinger

### References

Fischer, G. H., and Molenaar, I. (1995). Rasch Models - Foundations, Recent Developements, and Applications. Springer.

Mair, P., and Hatzinger, R. (2007). Extended Rasch modeling: The eRm package for the application of IRT models in R. Journal of Statistical Software, 20(9), 1-20.

Mair, P., and Hatzinger, R. (2007). CML based estimation of extended Rasch models with the eRm package in R. Psychology Science, 49, 26-43.

LRSM,LLTM

### Examples

#LPCM for two measurement points and two subject groups
#20 subjects, 2*3 items
G <- c(rep(1,10),rep(2,10))                   #group vector
res <- LPCM(lpcmdat, mpoints = 2, groupvec = G)
res
summary(res)


[Package eRm version 1.0-2 Index]