rpf (version 1.0.14)

rpf.prob: Map an item model, item parameters, and person trait score into a probability vector

Description

This function is known by many names in the literature. When plotted against latent trait, it is often called a traceline, item characteristic curve, or item response function. Sometimes the word 'category' or 'outcome' is used in place of 'item'. For example, 'item response function' might become 'category response function'. All these terms refer to the same thing.

Usage

rpf.prob(m, param, theta)

Value

a vector of probabilities. For dichotomous items, probabilities are returned in the order incorrect, correct. Although redundent, both incorrect and correct probabilities are returned in the dichotomous case for API consistency with polytomous item models.

Arguments

m

an item model

param

item parameters

theta

the trait score(s)

Examples

i1 <- rpf.drm()
i1.p <- rpf.rparam(i1)
rpf.prob(i1, c(i1.p), -1)   # low trait score
rpf.prob(i1, c(i1.p), c(0,1))    # average and high trait score