1. Coombs, C. H. A factorial study of number ability.Psychometrika, 1941,6, 161–189.
2. Holzinger, K. J. Preliminary report on Spearman-Holzinger unitary trait study, No. 6. Chicago: Statist. Lab., Dept. Educ., Univ. Chicago, 1935.
3. Otis, A. S. Otis group intelligence scale. Yonkers, N. Y.: World Book Company.
4. Philip, B. R. The measurement of attention.Studies in Psychology and Psychology, 1928, II, No. 1.
5. Seashore, C. E., Lewis, D., and Laetveit, J. G. Seashore measures of musical talent, 1939 Revision. Camden, N. J.: Educ. Dept. R. C. A. Mfg. Co., Inc.