If test equivalence holds and there are reasons to believe that the test score represents one underlying unitary faculty, then the question arises what that faculty may be. This question can only be answered by theory development and experimentation and not by blindly correlating tests with other tests or external criteria. If on the other hand the test score does admit of resolution into a plurality of sub-factors (to use Spearman's terminology), then the predictive validity of the tests is highly questionable. One does not know which factors are involved, nor what their contributions are to the final test score. If test equivalence does not hold, then further research is needed in order to understand why different groups of tests emerge. Usually researchers restrict themselves to pure naming without really explaining (see Thurstone, 1938). One should at least try to understand why more then one factor emerges in the first place.
Similar arguments hold for items. Till now no applications exist of item response models for reaction times to intelligence test items. However, the Rasch model has succesfully been applied in many cases. Positive results were obtained with easy tests (van der Ven, 1969) as well as with difficult tests (van den Wollenberg, 1997), but no evidence could be presented for the existence of any unitary power. In two cases (van der Ven 1992 a and b) serious attempts have been made how a violation of item equivalence could be explained. This gave rise to the coping hypothesis. In the case of certain items of a test the subjects resort to particular response categories as a kind of coping strategy prefering an easy, but incorrect solution, to the more difficult, but correct solution. These response categories are usually refered to as distractors. The fact that particular distractors are used in some of the items, but not in all may cause a violation of the assumption of unidimensionality (or better item equivalence). A second discriminative factor plays a role in the case of the items concerned. Additional to the tendency to find the real correct solution, these items also discriminate the subjects on the tendency to choose the distractor for a correct response.