The Planning of Experiments

A clinician or a medical laboratory worker who turns over the 250 pages of Professor R. A. FISHER's new book' and discovers that more than 200 pages are devoted to the discussion of the yield of plots of land at Rothamsted Experimental Station will be apt to conclude that the wQrk is no, concern of his. If, having a few minutes to spare, he samples -a few pages and discovers that, although a most sparing use is made of mathematical symbols or formulae, the author's argument requires a strenuous effort of attention to follow, his conclusion may be strengthened. It is, however, quite wrong. The adjecti-ves " great," " masterly," and " epoch-making " may properly be abandoned to the Sunday reviewers of novels and poems, but it is safe to say that this book is one of the most important contributions to scientific methodology of our generation. The lessons the author has to teach are well epitomized in the following quotations. " It is an essential condition of experimentation that the experimental material is known to be variable, but it is not known, in respect of any individual, in what direction his response to a given treatment will vary from the average. No direct allowance for this variability can, therefore, be made. The knowledge. which guides us in increasing the precision of an experiment is not a knowledge of the individual peculiarities of particular experimental units, such as plots ofland, experimental animals,coco-nut palms, or hospital patients, but a knowledge that there is less variation within certain aggregates of these than there is among different individuals belonging to different aggregates." (Page 78.) " We are usually ignorant which, out of innumerable possible factors, may prove ultimately to be the most important, though we may have strong presuppositions that some few of them are particularly worthy of study. We have usually no knowledge that any one factor will exert its effect independently of all others that can be varied, or that its effects are particularly simply related to variations in these other factors. On the contrary, if single factors are chosen for investigation, it is not because we anticipate that the laws of nature can be expressed with any particular simplicity in terms of these variables, but because they are variables which can be controlled or measured with comparative ease. If the investigator, in these circumstances, confines his attention to any single factor, we may infer either that he is the unfortunate victim of a doctrinaire theory as to how experimentation should proceed, or that the time, material, or equipment at his disposal are too limited to allow him to give attention to more than one narrow aspect of his problem." (Page 97.) "We may, by deliberately varying in each case some of the conditions of the experiment, achieve a wider inductive basis for our conclusions, witlout in any degree impairing their precision." (Page 107.)