Birnbaum's three-parameter logistic function has become a common basis for item response theory modeling, especially within situations where significant guessing behavior is evident. This model is formed through a linear transformation of the two-parameter logistic function in order to facilitate a lower asymptote. This paper discusses an alternative three-parameter logistic model in which the asymptote parameter is a linear component within the logit of the function. This alternative is derived from a more general four-parameter model based on a transformed hyperbola. (36pp.)
