EXPERIMENTS IN FIELD PLOT TECHNIC 



51 



plots protected by borders than of the single-row plots not so pro- 

 tected may actually be required for a given degree of plot variability. 

 Similarly, more replications may be required in a test of a large num- 

 ber of strains than in a test of a small number, as Montgomery" has 

 suggested. 



The number of replications required may be determined with a 

 fair degree of accuracy from the variability of the check plots. The 

 variability of the check plots in parts of the large fields used as com- 

 pared with the variability of the check plots in the whole fields shows 

 the importance of this point. In Table 25 are given the standard de- 



TABLE 25. RELATION OF PLOT VARIABILITY TO SIZE OF EXPERIMENT FIELD. 

 Check Plots in Wheat Variety Test 1920. 



*The four-range and eight-range sections are in order from west to east. 



viations of the yields of the check plots in the wheat variety test of 

 1920. The yields of the three interior rows of the check plots were 

 used in computing these constants. 



Twenty-four varieties could have been replicated four times in 

 the four ranges comprising any quarter of the field. As the probable 

 error of a single plot yield is 14.92 per cent we may conclude that the 

 probable error of the mean of four such yields would be about 7.46 

 per cent. But when 96 varieties must be tested, as they were in this 

 test, four replications require 16 ranges, and the probable error of the 

 mean yield becomes 8.47 per cent. A degree of precision which could 

 be attained with four replications in a test covering four ranges could 

 hardly be attained with five replications in a test covering sixteen 

 ranges. 



Corresponding data for the wheat variety test of 1921 are given 

 in Table 26. Although the variability in this experiment was much 

 lower, the relative variability of large and small experiment fields was 



