The more general questions of the theory of tests, their selection, evaluation, and technique of application and record, need not be considered here. The reader unfamiliar with these matters will find them fully treated in the various standard manuals of tests, and in numerous special articles and monographs referred to in the bibliography. There are, however, certain particular aspects of the theory and use of mental tests which have special importance for vocational psychology. These are: 1. The question of the degree to which proficiency in one respect or ability or test implies proficiency in others. 2. The degree to which these intercorrelations are revealed by preliminary trials and modified by continued practice. 3. The question of the significance of preliminary trials in revealing the relative abilities of individuals as these would be shown after all the individuals had acquired their maximum skill or practice level of proficiency; that is, the relation between momentary capacity and ultimate achievement. Attempts to intercorrelate mental or motor abilities as measured by laboratory tests have usually produced more or less irregular results. Some of the coefficients have been positive, some negative, but in only a few cases have many of them been large when the individuals tested have been chosen at random or with no deliberate intention of measuring only the extremes of the curve of distribution. Thus in a recent report of the correlations of abilities among several hundred adult individuals it is remarked that a certain test for logical memory is "one of the very best tests," partly because of "its high correlation with other tests" (an average correlation of .29). Two reasons are largely responsible for these low coefficients. The first is the fact that the measures correlated have usually been initial trials, or at most averages of a very few trials. This means great individual variability and considerable consequent unreliability of the data. A more important factor, perhaps, is the fact that these preliminary trials do not necessarily represent the final capacities of the individuals. They are determined by a host of incidental or accidental influences and reveal only momentary ability, not ultimate capacity. There is every reason for expecting to find positive correlation of "desirable" traits, and we may well expect to find this increasingly true the more our measures test the final limits of capacity in the various tests. In other words, the only real correction for unreliable measures is to be made by continuing the test until the individual has reached the limit of practice in it.