Effects of different measurement scales on the variability and predictive validity of the "two-component" model of the theory of planned behavior in the exercise domain

Limited measurement variability may reduce the ability of the theory of planned behavior (TPB) to predict desirable health behaviors. The primary purpose of the present study was to examine the effects of five different measurement scales on the variability and predictive validity of the TPB in the exercise domain. A secondary purpose was to test the utility of the "two-component" TPB model (i.e., affective and instrumental attitudes, injunctive and descriptive norms, and perceived control and self-efficacy). We randomly assigned 422 undergraduate students to complete one of the five measurement scales. Results showed that the four experimental scales significantly increased the variability in most TPB measures but did not improve the model's predictive validity. Moreover, support was found for the "two-component" model for attitude and subjective norm but not for perceived behavioral control. It was concluded that the standard 7-point scale is still the optimal measurement scale for the TPB and that the "two-component" model is superior to the traditional model in the exercise domain.