fourPL {petersenlab}

4-Parameter Logistic Curve.

Description

4-parameter logistic curve for item response theory.

Usage

fourPL(a = 1, b, c = 0, d = 1, theta)

Arguments

a

Discrimination parameter (slope).

b

Difficulty (severity) parameter (inflection point).

c

Guessing parameter (lower asymptote).

d

Careless errors parameter (upper asymptote).

theta

Person's level on the construct.

Details

Estimates the probability of item endorsement as function of the four-parameter logistic (4PL) curve and the person's level on the construct (theta).

Value

Probability of item endorsement (or expected value on the item).

See Also

doi:10.1177/0146621613475471

Other IRT: discriminationToFactorLoading(), itemInformation(), reliabilityIRT(), standardErrorIRT()

Examples

fourPL(b = 2, theta = -4:4) #1PL
fourPL(b = 2, a = 1.5, theta = -4:4) #2PL
fourPL(b = 2, a = 1.5, c = 0.10, theta = -4:4) #3PL
fourPL(b = 2, a = 1.5, c = 0.10, d = 0.95, theta = -4:4) #4PL