OPLM

The One Parameter Logistic Model (OPLM) is an Item Response Theory (IRT) model which combines the appealing properties of the Rasch model with the flexibility of the two-parameter logistic model.

In the OPLM item difficulty parameters are estimated and item discrimination parameters are imputed as known constants. For this model robust methods are developed to generate and test hypotheses pertaining to the magnitude of the discrimination indices. By relaxing the assumptions regarding the equality of the discrimination indices in the Rasch model, the OPLM model increases the applicability of the Rasch model. Imputing in stead of estimating the discrimination indices in the OPLM model preserves the powerful mathematical properties of the exponential model family, which are lost in the two-(or three-)parameter logistic models.

The software

OPLM is also the name of a computer program. OPLM is an MS-DOS application for analyzing item responses within the framework of the One Parameter Logistic Model (OPLM). It can handle items that are scored dichotomously or polychotomously. Because the OPLM model is a generalization of the Rasch model, the program can also perform Rasch analyses and Partial Credit model analyses.