72 MISSOURI AGR. EXP. STA. RESEARCH BULLETIN 49 



fective on fields highly variable in soil productivity than on more 

 uniform fields, and for similar reasons the method will probably be 

 found more effective in tests covering a rather large area than in tests 

 covering a smaller area. 



It is clear that the adjustment of yields by means of check plots 

 entails several serious disadvantages, and may increase experimental 

 error considerably. Not only is the yield of the check plot a far from 

 perfect measure of soil productivity for the check variety, but the pro- 

 ductivity of the same soil for other varieties may be decidedly dif- 

 ferent. The method is therefore more effective in tests of strains of 

 the same variety as the check, than in tests of different varieties. When 

 the yields of check plots are materially affected by factors not similarly 

 affecting the neighboring test plots, adjustment of yields will increase 

 experimental error. The check plots must therefore be effectively 

 protected from competition, border effect, mechanical errors, and the 

 like. Moreover, it is to be expected that the effectiveness of adjusting 

 yields will vary with the season, since the relative influence of soil 

 productivity on yield varies with the season. For example, in a season 

 in which winter injury is exceptionally severe, actual soil fertility may 

 have comparatively little to do with plot yields. Now, if the check 

 variety is hardy, its yields may vary with the soil fertility, but when 

 corresponding adjustments are made on the yields of tested varieties 

 limited in yield by winter injury, a decrease in the variability of repli- 

 cate plots is hardly to be expected. The same considerations apply 

 of course to yields limited by many other factors. 



But, although a multitude of objections may be made to the 

 theoretical bases of the practice of adjusting yields in variety tests, 

 and although in many cases it undoubtedly results in an increase rather 

 than a decrease in experimental error, the practice offers promise of 

 value and is worthy of further investigation. The effectiveness of 

 the adjustment of yields in the wheat variety test of 1920, in which 

 the variability of replicate test plots was reduced about 40 per cent, 

 is a demonstration of the possibilities of the method. An increase in 

 replication of plots involving the same increase in land and labor would 

 probably have reduced plot variability only about 7 per cent. A thor- 

 ough knowledge of the value and limitations of yield-adjustment by 

 means of check plots might enable us to reduce variability, at least in 

 some types of plot tests, much more effectively by this means than 

 by replication. The saving in area required is of particular significance 

 in preliminary tests if border rows must be used for the elimination 

 of competition, since in this case the area required for a large number 

 of replications is in many cases prohibitive. 



