A note on the 1-9 scale and index scale in ahp

This paper demonstrates that the 1-9 scale and the index scale used in the Analytical Hierarchy Process (AHP) can both be derived from the same scale-selection criteria. They are both accordant to Saaty's basic thoughts about the scale-selection in the AHP. These two different kinds of ratio scales are the results of the two different ways of applications of the Weber-Fechner psychophysical law. We believe that the index scale is preferable to the 1-9 scale in the theory and hope that the more use of the index scale could be encouraged in the future. But for the rich experiences of application, people can keep on using the existing 1-9 scale in the practice.