equateIRT (version 2.0)

est2pl: Item Parameter Estimates and Covariance Matrices of a Two-Parameter Logistic Model

Description

This dataset includes item parameter estimates and covariance matrices of a two-parameter logistic model applied to 5 simulated datasets with common items. The dichotomous item responses can be found in the dataset data2pl. See details for more information on the linkage plan.

Usage

data(est2pl)

Format

A list of length 2 with components:

coef

a list of length 5 containing the matrices of item parameter estimates. Each matrix presents 2 columns; the first column contains difficulty parameters and the second column contains discrimination parameters. See details for information on the parameterization used. Names of rows correspond to the names of the items.

var

a list of length 5 containing the covariance matrices of item parameter estimates.

Details

Every form is composed by 20 items and presents 10 items in common with adjacent forms. Furthermore, forms 1 and 5 present 10 common items. Use linkp to obtain a matrix with elements equal to the number of common items between different forms.

Item parameters are given under the parameterization used in the ltm package. Under this parameterization, the two-parameter logistic model is as follows $$\pi_i = \frac{\exp(\beta_{1i} + \beta_{2i} z)}{1 + \exp(\beta_{1i} + \beta_{2i} z)},$$ where $\pi_i$ denotes the conditional probability of responding correctly to the $i$th item given $z$, $\beta_{1i}$ is the easiness parameter, $\beta_{2i}$ is the discrimination parameter, and $z$ denotes the latent ability.

See Also

data2pl, linkp, modIRT

Examples

data(est2pl)
est2pl$coef
est2pl$var
linkp(coef = est2pl$coef)