NAEP Analysis and Scaling → Estimation of NAEP Score Scales → Item Scaling Models → The Two-Parameter Logistic Model The Two-Parameter Logistic ModelA two-parameter logistic (2PL) model is used for the short constructed-response items that were scored as correct or incorrect. The form of the 2PL model is the same as that of the 3PL model, with the cj parameter fixed at zero. The fundamental equation of the 2PL model defines the probability that a person whose score on scale k is characterized by the unobservable variable θk will respond correctly to item j as where
The probability of an incorrect response to the item is further defined as Last updated 14 July 2008 (DB) |