irtoys (version 0.1.5)

tif: Test information function

Description

Returns the test information function (TIF) of the 3PL (1PL, 2PL) model. The TIF is the sum of the item information functions (IIF) in a test, and indicates the precision of measurement that can be achieved with the test at any value of the latent variable, bein inversely related to measurement variance.

Usage

tif(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 TIF 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 TIF values

Details

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

See Also

plot.tif, iif

Examples

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