iif {irtoys} | R Documentation |
The item information function (IIF) for the 3PL model can be computed as
I(theta) = a^2frac{Q(theta)}{P(theta)}<=ft[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.
A common use of this function would be to obtain a plot of the IIF.
iif(ip, x = NULL)
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 IIF will be evaluated. If not given, 99 values spaced evenly between -4 and +4 will be used, handy for plotting. |
A list of:
x |
A copy of the argument x |
f |
A matrix containing the IIF values: persons (values of (x ) as rows and items as columns |
Ivailo Partchev
data(Scored) p.2pl <- est(Scored, model="2PL", engine="ltm") plot(iif(p.2pl[1:3,]))