EXPERIMENTS IN FIELD PLOT TECHNIC 



71 



justed according to the Kherson check and Red Rustproof strains 

 according to the Red Rustproof check. 



Value and Limitations of Adjusting Yields by Means of Check 

 Plots. The effect on plots variability of adjusting yields by means of 

 check plots in all of the tests is shown in summary form in Table 40. 

 The variability of the test plots was reduced by adjustment in three 

 tests and was increased in the other five. It is noteworthy that the 

 three tests in which plot variability was reduced by adjustment were 

 characterized by high plot variability, as indicated by the standard 

 deviation of check plots, while the tests in which adjustment -was not 



TABLE 40. SUMMARY OF RELATIVE VARIABILITY OF ACTUAL AND ADJUSTED 

 YIELDS OF INTERIOR Rows IN ALL TESTS. 



Test 



Number Number Average deviation 



Season of var- or rep- Actual Adjusted 



eties or lica- yields yields 



strains tions 



* Adjustment by Kherson check. 



effective were in general low in plot variability. In 1919 adjustment 

 was quite effective in reducing variability in the oats variety test, while 

 it increased variability in the oats strain test, which was conducted on 

 the same field and similarly handled in every way. In fact, conditions 

 were considered more favorable for the effectiveness of the practice 

 in the strain test than in the variety test, for the check plots were closer 

 together and 12 of the 15 strains tested were taxonomically identical 

 with the check. But the standard deviation of check plots on the 

 part of the field on which varieties were grown was almost twice as 

 great as on the part of the field on which the strains were grown. Ap- 

 parently the high variability of the plots in the variety test was caused 

 in large part by differences in actual soil productivity which were 

 largely counteracted by the adjustment of yields, while there was 

 little variation in soil productivity in the strain test and such plot 

 variability as occurred was largely due to other factors. In general 

 therefore the adjustment of yields will probably be found more ef- 



