stepwiseIt {eRm} R Documentation

Stepwise item elimination

Description

This function eliminates items stepwise according to one of the following criteria: itemfit, Wald test, Andersen's LR-test

Usage

## S3 method for class 'eRm'
stepwiseIt(object, criterion = list("itemfit"), alpha = 0.05,
verbose = TRUE, maxstep = NA)


Arguments

 object Object of class eRm. criterion List with either "itemfit", "Waldtest" or "LRtest" as first element. Optionally, for the Waldtest and LRtest a second element containing the split criterion can be specified (see details). alpha Significance level. verbose If TRUE intermediate results are printed out. maxstep Maximum number of elimination steps. If NA the procedure stops when the itemset is Rasch homogeneous.

Details

If criterion = list("itemfit") the elimination stops when none of the p-values in itemfit is significant. Within each step the item with the largest chi-squared itemfit value is excluded.

If criterion = list("Waldtest") the elimination stops when none of the p-values resulting from the Wald test is significant. Within each step the item with the largest z-value in Wald test is excluded.

If criterion = list("LRtest") the elimination stops when Andersen's LR-test is not significant. Within each step the item with the largest z-value in Wald test is excluded.

Value

The function returns an object of class step containing:

 X Reduced data matrix (bad items eliminated) fit Object of class eRm with the final item parameter elimination it.elim Vector contaning the names of the eliminated items res.wald Elimination results for Wald test criterion res.itemfit Elimination results for itemfit criterion res.LR Elimination results for LR-test criterion nsteps Number of elimination steps

LRtest.Rm, Waldtest.Rm, itemfit.ppar

Examples


## 2pl-data, 100 persons, 10 items
set.seed(123)
X <- sim.2pl(500, 10, 0.4)
res <- RM(X)

## elimination according to itemfit
stepwiseIt(res, criterion = list("itemfit"))

## Wald test based on mean splitting
stepwiseIt(res, criterion = list("Waldtest","mean"))

## Andersen LR-test based on random split
set.seed(123)
groupvec <- sample(1:3, 500, replace = TRUE)
stepwiseIt(res, criterion = list("LRtest",groupvec))



[Package eRm version 1.0-2 Index]