The method of constant stimuli with three rating categories and the use of Rasch models

Logistic Models support an alternative general solution to compute the point ofsubjective equality (Vidotto et al., I modelli simple logistic e rating scale nella determinazionedel punto di eguaglianza soggettivo: una nuova prospettiva per il metodo degli stimolicostanti, 1996) when the Method of Constant Stimuli is used (Bock and Jones, The Measurementand Prediction of Judgment and Choice, 1968). The Extended Logistic Models (Andrich,Appl. Psychol. Meas. II:581–594, 1978) offer the theoretical frame to compute individual andgeneral thresholds when the method of constant stimuli is applied using a forced choice withmore than two alternatives. As an example of the advantages of the application of this procedure,we show a data-set derived from an experiment on rhythm perception (Maestrini, Lapercezione del cambiamento nei ritmi uditivi, 2003), where two groups of experimental andnaïve subjects were asked to judge whether the listened rhythm was constant, accelerated ordecelerated. We have computed individual and general thresholds differentiating constancyfrom both acceleration and deceleration for all the different experimental conditions. Themain advantage of this solution, compared to traditional psychophysical techniques, is notonly related to better estimates of the individual point of subjective equality. The improvementcan be summarized in the available fit tests to verify the agreement between the modelboth for stimuli and subjects.