1. Bradstreet, C. M. P., and Taylor, C. E. D. (1962). Mth. Bull. Minist. Hlth. Lab. Serv., 21, 96.
2. Buchanan, G. (1927). Spec. Rep. Ser. med. Res. Coun. (Lond.), 113.
3. Davidson, L. S. P., and Smith, J. (1936). Quart. J. Med., 5, 263.
4. Grist, N. R. (1956). Scot. med. J., 1, 173.
5. Henderson, R. J. (1967). Lancet, 2, 353.