1. Siegel, S. (1956).Nonparametric statistics for the behavioural sciences.New York, McGraw-Hill.
2. Whitfield, W. A. D. (1974). The soils of the National Vegetable Research Station, Wellesbourne.Report of the National Vegetable Research Station for 1973,21–30.
3. Wurr, D. C. E., Fellows, J. R., Pittam, A. J. and Arnold, G. A. (1985). Crisphead lettuce. Effect of temperature during plant raising on head weight at maturity.National Vegetable Research Station Annual Report for 1984,112–13.