De Gruijter, Dato N. M.De Jong, John H. A. L.2011-08-292011-08-291991de Gruijter, Dato N & de Jong, John H. (1991). Item-rest regressions, item response functions, and the relation between test forms. Applied Psychological Measurement, 15, 25-34. doi:10.1177/014662169101500104doi:10.1177/014662169101500104https://hdl.handle.net/11299/114017Levine (1982) used item-rest regressions for the estimation of item parameters, and this relationship was exploited in this research in the context of vertical equating. Results from a simulation and an empirical dataset were used to demonstrate that item-rest regressions were useful in verifying the relationship between two tests obtained from item parameter estimates. It is shown that in vertical equating designs the Rasch model cannot replicate the relationship between tests at the lower score levels when guessing is present. At higher score levels, however, the correct transformation function can be estimated, irrespective of the IRT model used. Index terms: equating, guessing parameter, item response functions, item-rest regression, Rasch model.enArticle