We present a method for studying experimental data based on a psychometric model, the "Rasch model" (Rasch, 1966;Thissen & Steinberg, 1986). Weillustrate the method with the use of a data set in the field of concept research. More specifically, we investigate whether a conjunctive concept can be seen as an additive combination of its constituents, High correlations between model and data are obtained, but a formal goodness-of-fit test indicates that the model does not completely account 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 is concluded that our ideas, as incorporated in the model, adequately summarize the data. More generally, this research illustrates that applying the Rasch model and altering it according to one's hypotheses is an excellent way to analyze experimental data.
501In the perception literature, considerable attention has been paid to thefuzzy logic model ofperception (FLMP; see, e.g., Massaro & Hary, 1986; Oden, 1977). Recently, Crowther, Batchelder, and Hu (1995) have established the formal equivalence between this model and a particular psychometric model-namely, the Rasch model (Rasch, 1966;Thissen & Steinberg, 1986). In this paper, we explore the consequences ofthis linkage and apply the Rasch model (or, equivalently, the FLMP) to data on conjunctive concepts (see, e.g., Hampton, 1987Hampton, , 1988Storms, De Boeck, Van Mechelen, & Geeraerts, 1993;Storms, De Boeck, Van Mechelen, & Ruts, 1996). We first present the FLMP model, followed by the (equivalent) Rasch model. We then apply the model to empirical data and discuss its analysis. In the example, both models lead to the same conclusions, although some extra information is provided in the Rasch formulation.
THE FUZZY LOGIC MODEL OF PERCEPTIONSuppose we perform a letter recognition experiment (e.g., Oden, 1979) and ask participants whether a stimulus 0i should be classified as a G or as a Q. Furthermore, suppose that c l i indicates the degree to which aspectj is present in stimulus°i: If aspectj is present in the letter G, the evidence for G with respect to aspectj is equal to c l i ; otherwise, it is equal to I -Clio The same reasoning applies for the letter Q. For example, given that both G and Qare made up ofan oval and a line, the first aspect might be "oval is open on the right." The evidence in 0i for G with respect to this first aspect is Cli; for Q, it is I -Cli' Similarly, the second aspect might be "line is horizontal."The authors thank Iven Van Mechelen, Norman Verhelst, and Johan Wagemans for their useful comments. Correspondence should be addressed to T. Verguts, Department of Psychology, Tiensestraat 102, B-3000 Leuven, Belgium (e-mail: tom.verguts@psy.kuleuven.ac.be).The evidence for G and Qin stimulus°i is then c2i and 1 -c2i' respectively.To continue with our example: The FLMP predicts that the probability of stimulus 0i to be classified as G [formally, Pr (o,...