WHY FACTOR-ANALYSIS OFTEN IS THE INCORRECT MODEL FOR ANALYZING BIPOLAR CONCEPTS, AND WHAT MODEL TO USE INSTEAD

Factor analysis of data that conform to the unfolding model often resu lts in an extra factor. This artificial extra factor is particularly i mportant when data that conform to a bipolar unidimensional unfolding scale are factor analyzed. One bipolar dimension is expected, but two factors are found and often are interpreted as two unrelated dimension s. Although this extra factor phenomenon was pointed out in the early 1960s, it still is not widely recognized. The extra factor phenomenon in the unidimensional case is reviewed here. A numerical illustration is provided, and a number of diagnostics that can be used to determine whether data conform to the unidimensional unfolding model better tha n to the factor model are discussed. These diagnostics then are applie d to an empirical example.