ABSTRACT
Logistic model (Logistic Model) proposed by Birnbaum is the most widely used IRT model. According to the parameter numbers of different characteristic function, the model can be divided into one-Parameter Logistic Model (1PLM), twoParameter Logistic Model (2PLM) and three-Parameter Logistic Model (3PLM). Three Parameters Logistic Model (3PLM) is showed as follows:
P c c
Dai i ( ) ( )b+
−
+ − 1
Among them, Pi (θ) represents the probability which can answer questions correctly of number Figure 1. Character curve of logistic model (3PLM).
