Measurement with multiple indicators and psychophysical scaling in the context of Fishbein and Ajzen's theory of reasoned action

Two experiments were carried out to test the theory of reasoned action of Fishbein and Ajzen. The measurements were done using two category scales and two psychophysical scales. No consistent difference in results was found between the four modalities. However, if the latter were used as multiple indicators for the model components in a latent variables model, the parameter values did increase as compared to those in single indicator models. A multitrait-multimethod model allowed for correction of method effects. Though the results of the multiple indicator models did not differ significantly from the results of the single indicator models, the increase in explained variance of intention by 17% in the first experiment and 15% in the second experiment can not be ignored in applied research. Psychophysical scales are attractive because they are well suited for use as multiple indicators.