In the fall of 1948 the five-part Naval Postgraduate School Aptitude Test Form WNPF was administered at shore installations and on ships to applicants for admission to the Postgraduate School. Test data for those later admitted to the School have now been compared with similar data for those who were not selected, to allow comparisons of the applicant and admitted groups and to estimate the effects of the present selection process on the magnitudes of the test validities being obtained in the experimental test-development program being carried on at the Postgraduate School. It was found that the greatest reduction in range of talent had occurred for the Mathematics Aptitude part of the test, and, as a consequence, that the validity of this part was most markedly affected by the selection process. Lesser effects were noted for the Reading Comprehension, Spatial Intersections, and Physics parts of the test. No reduction in range of talent was observed for the Verbal Antonyms part, and since it seems probable that the validity of this test is not seriously affected by the selection process, its observed low validity may be concluded not to be a result of rigorous selection on verbal ability. It was concluded that in the future interpretation of test validities, the fact must be taken into account that the present method of selection restricts the range of ability in mathematics aptitude somewhat more than in other areas. Observed validities for mathematics tests are therefore likely to have been lowered more by the selection process than those of other tests involved in the study.