  When applied to contingency tables, dual scaling and correspondence are
mathematically equivalent methods. For the analysis of rating data, however,
the methods differ. To a large extent this is due to differences in
preprocessing of the data. In particular, in dual scaling, ratings are either
transformed to rank order, or to successive category data before applying a
customised dual scaling approach. In correspondence analysis, on the other
hand, a so-called doubling of the original ratings is applied before applying
the usual correspondence analysis formulas. In this paper, we consider these
differences in detail. We propose a dual scaling variant that can be applied
directly to the ratings and we compare theoretical as well as practical
properties of the different approaches.
