ANALYZING EXPERIMENTAL-DATA USING THE RASCH MODEL

We present a method for studying experimental data based on a psychome tric model, the ''Rasch model'' (Rasch, 1966; Thissen & Steinberg, 198 6). We illustrate the method with the use of a data set in the field o f concept research. More specifically, we investigate whether a conjun ctive concept can be seen as an additive combination of its constituen ts. High correlations between model and data are obtained, but a forma l goodness-of-fit test indicates that the model does not completely ac count for the data. We then alter the Rasch model in such a way as to capture our idea of why the model deviates from the data. This results in higher correlations and a strong increase in goodness-of-fit. It i s concluded that our ideas, as incorporated in the model, adequately s ummarize the data. More generally, this research illustrates that appl ying the Rasch model and altering it according to one's hypotheses is an excellent way to analyze experimental data.