trf {irtoys}R Documentation

Test response function

Description

Returns the test response function (TRF) of the 3PL (1PL, 2PL) model. The TRF is the sum of the item response functions (IRF) in a test, and represents the expected score as a function of the latent variable theta.

A common use of this function would be to obtain a plot of the TRF.

Usage

trf(ip, x = NULL)

Arguments

ip Item parameters: a matrix with one row per item, and three columns: [,1] item discrimination a, [,2] item difficulty b, and [,3] asymptote c.
x The values of the latent variable (theta in the equation above), at which the IRF will be evaluated. If not given, 99 values spaced evenly between -4 and +4 will be used, handy for plotting.

Value

A list of:

x A copy of the argument x
f A vector containing the TRF values

Author(s)

Ivailo Partchev

See Also

plot.trf, irf

Examples

data(Scored)
p.2pl  <- est(Scored, model="2PL", engine="ltm")
plot(trf(p.2pl))

[Package irtoys version 0.1.1 Index]