equateIRT (version 2.5.1)

import.ltm: Import Item Parameters Estimates and Covariance Matrices from IRT Software

Description

Import estimated item parameters and covariance matrix from the R packages ltm and mirt, and from external software IRTPRO and flexMIRT.

Usage

import.ltm(mod, display = TRUE, digits = 4) 
import.mirt(mod, display = TRUE, digits = 3) 
import.irtpro(fnamep, fnamev = NULL, fnameirt = NULL, display = TRUE, digits = 2) 
import.flexmirt(fnamep, fnamev = NULL, fnameirt = NULL, display = TRUE, digits = 2)

Value

A list with components

coef

item parameter estimates.

var

covariance matrix of item parameter estimates.

Arguments

mod

output object from functions rasch, ltm, or tpm of the ltm package or from function mirt of the mirt package.

display

logical; if TRUE coefficients and standard errors are printed.

digits

integer indicating the number of decimal places to be used if display is TRUE.

fnamep

name of the file containing the estimated item parameters. Typically, -prm.txt.

fnamev

name of the file containing the covariance matrix of the estimated item parameters. Typically, -cov.txt.

fnameirt

name of the file containing additional information to link item parameters with the covariance matrix. Typically, -irt.txt.

Author

Michela Battauz

Details

Item parameters are imported with the parameterization used by the software to estimate the IRT model. The usual IRT parameterization can be obtained later by using function modIRT.

Since package version 2.5.0 this function can be skipped if the item parameters are estimated with packages ltm or mirt. See modIRT.

References

Battauz, M. (2015). equateIRT: An R Package for IRT Test Equating. Journal of Statistical Software, 68, 1--22.

Cai L. (2013). FlexMIRT version 2: Flexible Multilevel Multidimensional Item Analysis and Test Scoring [Computer Software]. Chapel Hill, NC: Vector Psychometric Group.

Cai, L., du Toit, S. H. C., Thissen, D. (2011). IRTPRO: Flexible, multidimensional, multiple categorical IRT modeling [Computer software]. Chicago: Scientific Software International.

Chalmers, R. P. (2012). mirt: A Multidimensional Item Response Theory Package for the R Environment. Journal of Statistical Software, 48, 1--29.

Rizopoulos, D. (2006). ltm: an R package for latent variable modelling and item response theory analyses. Journal of Statistical Software, 17, 1--25.

See Also

modIRT

Examples

#====================
# from package ltm
library(ltm)

# one-parameter logistic model
mod1pl <- rasch(LSAT)
est.mod1pl <- import.ltm(mod1pl)
est.mod1pl

# two-parameter logistic model
mod2pl <- ltm(LSAT ~ z1)
est.mod2pl <- import.ltm(mod2pl)
est.mod2pl

#====================
# from package mirt
library(mirt)

# one-parameter logistic model
data(LSAT, package = "ltm")
val <- mirt(LSAT, 1, SE = TRUE, pars = "values")
cnstr <- val[val$name == "a1",]$parnum
mod1pl.m <- mirt(LSAT, 1, SE = TRUE, SE.type = 'Richardson', constrain = list(cnstr))
est.mod1pl.m <- import.mirt(mod1pl.m, digits = 4)
est.mod1pl.m

# two-parameter logistic model
data(LSAT, package = "ltm")
mod2pl.m <- mirt(LSAT, 1, SE = TRUE, SE.type = 'Richardson')
est.mod2pl.m <- import.mirt(mod2pl.m, display = FALSE)
est.mod2pl.m