The Interpretation of Experimental Results

Abstract

Attention is drawn to the need for caution in interpreting experimental results.Frequency curves are discussed, chiefly from the point of view of their bearing on the legitimacy of averaging results.The method of calculating probable error is described and its meaning explained.The application of probable error methods to questions of sampling for analysis, to field experiments and to feeding experiments, are illustrated by instances.The probable error of one animal on a fattening ration is found to be about 14 per cent, of the live-weight increase produced, from which it is calculated that to obtain a precision of 10 per cent, in an ordinary feeding experiment 29 animals must be fed on each ration.The probable error of field experiments is investigated by two independent methods, and found to be about 5 per cent, of the crop. This figure is shown to be independent of the size of the plot employed, provided this is acre or larger. A table is given showing the number of duplicate plots which must be employed to give any desired precision in the result.It is also suggested that accurate results may be obtained by employing large numbers of very small plots, even as small as one square yard. This method is useful for nursery work in testing the cropping power of new varieties of cereals where very little seed is available.

The Relation of Size, Shape, and Number of Replications of Plats to Probable Error in Field Experimentation1

“…On the area which forms the baSis of these studies, no plat that wss much below one-twentieth of an acre in area or that had its greatest dimension in the direotion of least variation gave a coeffioient of variability of less than 2.5 per cent. The effect of using a unit of comparison composed of systematioally distributed parts is shown in This conclusion is also verified by the work of Montgomery (1911), Morgan (1908), Wood and stratton (1910), Egorov (1909), Hall and Russel (1911), Mortensen (1911), Gre~ goire (1913), Olmstead (1914), and Mercer and Hall (1911).…”

The resisted rise of randomisation in experimental design: British agricultural science, c.1910–1930

“…This is, or should be, the method followed by the agriculturalist. (Wood and Stratton 1910) Here then is a further goal, C) to manage widely variable environmental conditions, whichit should be noted -is not a problem that is confined solely to sciences in the field, but can be just as important in the lab (Ankeny et al 2014). In the same article Wood also reflected on the other values of field experiments aside from the collection of new data; "By laying down such local plots and meeting farmers on them to inspect and discuss the results, the staffs of the various institutions have been brought into touch with the agricultural public, and a mutual understanding has resulted."…”

R. A. Fisher and his advocacy of randomization

“…In what may be considered a landmark paper in English agriculture, Wood and Stratton (1910), in ''The Interpretation of Experimental Results,'' applied the astronomerÕs techniques of estimating error to several sets of agricultural data. (Russell, 1966, pp.…”