b Parameter

The b parameter is an item response theory (IRT)–based index of item difficulty. As IRT models have become an increasingly common way of modeling item response data, the b parameter has become a popular way of characterizing the difficulty of an individual item, as well as comparing the relative difficulty levels of different items. This entry addresses the b parameter with regard to different IRT models. Further, it discusses interpreting, estimating, and studying the b parameter.

Figure 1 Item Characteristic Curves for Example Items, One-Parameter Logistic (1PL or Rasch), Two-Parameter Logistic (2PL), and Three-Parameter Logistic (3PL) Models