1. Pugh, L. G. C. E., Brit. Med. J., 621 (1962).
2. Poulton, E. C., Catton, M. J., and Carpenter, A., Brit. J. Indust. Med., 21, (in the press).
3. Siegel, S., Nonparametric Statistics for the Behavioral Sciences (McGraw-Hill, New York, 1956).
4. MacFarland, R. A., J. Comp. Psychol., 24, 189 (1937).
5. Tichaeur, E. R., Ergonomics, 6, 51 (1963).