irtoys (version 0.1.5)

trf: 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 $\theta$.

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

Details

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

See Also

plot.trf, irf

Examples

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