Item parameters: the output of <code>est</code>, or a 3-column matrix 
corresponding to its first element, <code>est</code>.

The item(s) for which the information function is computed. 
If NULL (the default), irf for all items will be returned

items

The values of the latent variable ($\theta$ in the equation
above), at which the IIF will be evaluated. If not given, 99 values spaced
evenly between -4 and +4 will be used, handy for plotting.

The item information function (IIF) for the 3PL model can be computed as
$$I(\theta) =
a^2\frac{Q(\theta)}{P(\theta)}\left[\frac{P(\theta)-c}{1-c}\right]^2,$$ where
$\theta$ is the value of the latent variable for a person, $a$ is
the discrimination parameter for the item, $P$ is the IRF for the person
and item, and $Q=1-P$. For the 1PL and 2PL models, the expression
reduces to $a^2PQ$.

models

A collection of functions useful in learning and practicing IRT,
which can be combined into larger programs. Provides basic CTT analysis,
a simple common interface to the estimation of item
parameters in IRT models for binary responses with three different programs
(ICL, BILOG-MG, and ltm), ability estimation (MLE, BME, EAP, WLE, plausible
values), item and person fit statistics, scaling methods (MM, MS, Stocking-Lord,
and the complete Hebaera method), and a rich array of parametric and
non-parametric (kernel) plots. Estimates and plots Haberman's interaction model
when all items are dichotomously scored.

iif: Item information function

Description

Usage

Arguments

Value

Details

See Also

Examples