msdprob: Rating Category Probabilities
In msd: Method of Successive Dichotomizations

Description Usage Arguments Details Value Note Author(s) Examples

View source: R/msd.R

Estimates the probability of observing each rating category given a set of ordered rating category thresholds.

1	msdprob(x, thresholds)

`x`	a real number or a vector of real numbers with no NA representing a set of person minus item measures.
`thresholds`	a numeric vector of ordered rating category thresholds with no NA.

It is assumed that thresholds partitions the real line into length(thresholds)+1 ordered intervals that represent the rating categories.

A matrix of probabilities where each of the length(thresholds)+1 rows represents a different rating category (lowest rating category is the top row) and each of the length(x) columns represents a different person minus item measure.

msdprob can be used to create probability curves, which represent the probability of rating an item with each rating category as a function of the person measure minus item measure (see Examples).

Chris Bradley (cbradley05@gmail.com)

# Simple example
p <- msdprob(c(1.4, -2.2), thresholds = c(-1.1, -0.3, 0.5, 1.7, 2.2))

# Plot probability curves — each curve represents the probability of
# rating an item with a given rating category as a function of the
# person measure minus item measure.
x <- seq(-6, 6, 0.1)
p <- msdprob(x, thresholds = c(-3.2, -1.4, 0.5, 1.7, 3.5))
plot(0, 0, xlim = c(-6, 6), ylim = c(0, 1), type = "n",
    xlab = "Person minus item measure", ylab = "Probability")
for (i in seq(1, dim(p)[1])){
  lines(x, p[i,], type = "l", lwd = "2" , col = rainbow(6)[i])
}