His book "The Nature of Human Values" (1973) and the Rokeach Value Survey (see values scales), which the book served as the test manual for, occupied the final years of his career. In it, he posited that a relatively few "terminal human values" are the internal reference points that all people use to formulate attitudes and opinions, and that by measuring the "relative ranking" of these values one could predict a wide variety of behavior, including political affiliation and religious belief. This theory led to a series of experiments in which changes in values led to measurable changes in opinion for an entire small city in the state of Washington.