The three-parameter logistic model is a model where the lower asymptote parameter c is added to the 2PLM
ThreePLM(a, b, c, theta)
Returns a numeric vector of probabilities between c and 1, representing the probability of a correct response given the ability level theta. The probability is calculated using the formula: \(P(\theta) = c + \frac{1-c}{1 + e^{-a(\theta-b)}}\)
slope parameter
location parameter
lower asymptote parameter
ability parameter