1909.] The Experimental Error in Field Trials. 369 



obtaining a closer result is to multiply the experiments, 

 either by repeating them year after year or by increasing the 

 number of plots, preferably both, because there may be con- 

 stant differences in the soil, while the season also may induce 

 variations in the effect of the treatment. The first step, how- 

 ever, is to multiply the number of plots set aside for each kind 

 of treatment; taking five plots irregularly distributed about 

 the field, we shall obtain in a single year a result that is as 

 accurate as need be, except for special variations induced by 

 the character of the season. Of course this means a consider- 

 able increase in the amount of work attached to the experi- 

 ment. For example, instead of six plots each of half an acre, 

 we ought to take thirty plots of a tenth of an acre, six different 

 kinds of plots and five of each kind; every plot would also 

 need to be harvested and recorded separately. Such a form of 

 experiment is necessary if small differences are to be brought 

 out, as, for example, the differences that exist between the 

 various kinds of barley usually grown in this country. 



Increased accuracy is not to be obtained by increasing the 

 size of the plots ; it is questionable whether irregularities of 

 soil are likely to be more or less pronounced on large plots, 

 and with very large plots one new source of error is always 

 introduced — the difficulty of getting the cultivations, sowing, 

 harvesting, &c, of all the plots carried through in the same 

 day. As long as the plots are above /<j acre size does not 

 matter much, and the size that is most convenient for the 

 handling of the crop, its weighing, storage, threshing, &c, 

 should be selected, always remembering that it is by the 

 number of plots only that the error can be reduced. 



It is altogether wrong to take large plots and then select 

 small areas within the plot for weighing. Such a proceeding 

 introduces the most fatal error of all, a selection biased by 

 the preconceived opinion of the experimenter. It is also 

 incorrect to make allowances for missed plants, as is some- 

 times done by counting the number of roots and calculating 

 what the weight per acre would have been had there been a 

 perfect plant. Most manures affect the texture of the soil, 

 and therefore the number of plants which establish them- 

 selves is one of the factors in the result that is directly affected 

 by the manure. 



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