Categorical scales provide many advantages in educational evaluations, which contrast with the delicate statistical analysis that their measurements require. Metric properties have not always been treated properly, sometimes because of ignorance, others by the absence of common criteria about which statistical techniques are more appropriate. In this work, we point out some of the reasons for this confusing panorama we also provide little known tips on analysis in educational research, with a threefold aim: an improvement on the metric of the scales, the search for internal data dimensions, and to study the obtained responses from previously established dimensions. To achieve the first one, it is proposed the Zipf's law, certainly a very interesting suggestion to transform the metric of the frequency categorical scales. In order to get the second, some extensions to the factorial analysis are suggested. Finally, and through intuitive example, we propose a Conjoint Analysis referred to a particular educational evaluation.

Scales, Likert scales, factorial analysis, conjoint analysis, Zipf’s law.