IzstarMix

Author: Sandip Sinharay
Contact: ssinharay@pacificmetrics.com
Description: In a report for the Council of Chief State School Officers, Olson and Fremer (2013) recommended the use of person-fit assessment, in addition to other methods, to detect irregularities in answering behavior of the test takers. lzstarMix is a function written in R (R Core Team, 2015) to compute the lz statistic for mixed-format tests (Drasgow, Levine, & Williams, 1985; Sinharay, 2015b). The function also computes the lz statistic for mixed-format tests that was recently suggested by Sinharay (2015a). The null distribution of lz is generally much closer than that of lz to the standard normal distribution for mixed-format tests. The function lzstarMix currently works when the dichotomous items are modeled using the three-parameter logistic (3PL) model, and the polytomous items are modeled using the generalized partial credit model (GPCM; Muraki, 1992). The input to the function consists of the item-parameter estimates, and the item scores and estimated abilities (maximum likelihood estimate or weighted likelihood estimates) of several examinees. The output are lz and lz for the examinees. The function can be used for tests that include only dichotomous items, in which case lz for mixed-format tests (Sinharay, 2015a) becomes identical to lz for dichotomous items (Snijders, 2001), or only polytomous items.
Analyses: Attitude Scaling; DIF; Item & Test Analysis; Latent Class Models; Person Fit/Cheating; Reliability; Simulation/Resampling; Test Construction & Administration
System Requirements: Windows; Mac; Linux
Measurement Model: IRT/Rasch
License Type: Open-Source
Documentation: User manual, sample data sets