Russell, W. M. S. , Burch, R. L. 1992. The Principles of Humane Experimental Technique - Special Edition. Wheathampstead, UK, Universities Federation for Animal Welfare.

The wages of inhumanity are paid in ambiguous or otherwise unsatisfactory experimental results. The authors introduce the concept of the 3 Rs, i.e. Replacement, Reduction, Refinement. Desirable as replacement is, it would be a mistake to put all our humanitarian eggs in this basket alone. The progress of replacement is gradual, nor is it ever likely to absorb the whole of experimental biology. Refinement may reach such a pitch that a given procedure employing animals is absolutely humane, but in any given field there is bound to be a latent period before such success is attained....reduction remains of great importance, and of all modes of progress it is the one most obviously, immediately, and universally advantageous in terms of efficiency. ... If, for instance, we can literally remove any unwanted source of variance, we reap our reward at once in smaller residual variance, greater precision, and hence fewer experimental animals. Refinement means any decrease in the incidence or severity of inhumane procedures applied to those animals which still have to be used (p. 64). Its object is simply to reduce to an absolute minimum the amount of distress imposed (p. 134).*Originally published in 1959, Methuen & Co. London, UK