Or if the assumption of a common factor be rejected, the present evidence tends strongly to support our earlier conclusion concerning the positive correlation between desirable mental functions. Some form of the doctrine of "general ability," at any rate, seems to be supported. But the conclusion seems to call for the qualification that "general ability" shall have reference to _final capacity_ rather than to _momentary performance_, if the correlations are to be high. If each individual be given the opportunity to attain his limit of efficiency, his highest level of performance, then, when these final limits are reached, individuals who excel their fellows in one type of work will also tend to excel in other types of work.
The theory and practice of tests has in the past been too content to rest its claims on the meager results of a few preliminary samplings of an individual's ability. The fact that, even when a great variety of such samplings of a given individual are aggregated and balanced off against one another, few results of real diagnostic value are achieved should be sufficient warning against this tendency. My conviction is that for this purpose we shall find it necessary to determine the individual's "limit of practice" in the various tests before we shall secure diagnostic results which will be verified by the individual's subsequent achievement in daily life. We should know much more than we now know concerning the tendency and meaning of such correlations as show close relation between initial performance and ultimate capacity. This is particularly true if we wish to extend the method of tests beyond educational diagnosis and to use them as a means of vocational guidance or of industrial selection. For educational diagnosis we wish primarily to know what kind of practice the individual most needs. For vocational and industrial purposes we need rather to know what limits the individual can eventually reach, in given kinds of performance, as the result of practice, and to what degree his present equipment of incentive renders probable the actual achievement of this limit.
On the question of the significance of preliminary trials and the effects of practice on the relative standing of individuals in their group, there are important facts to be considered. In the direct application of mental tests it has too often been assumed that the actual performance of an individual, in one or a dozen trials at a given task, is in some way or other significant of that individual's final capacity in such work. It is true that several investigators have studied the effects of practice on individual differences. These workers were interested above all in questions as to relative rate of improvement, or amount or permanence of gain. Such studies have produced suggestive results, although they have been based, for the most part, on records of only a few subjects or on relatively few practice trials.