Experimental Ei^ror in Crop Tests 
51 
quadrate means, for the several sorts of wheat, deviated 2.2 
per cent from the average of the dupHcate plat yields. 
When the quadrates from each plat were grouped into 
sets of five and ten each, there was considerable variation 
in yield between the separate groups, which suggests that 
not less than 20 quadrates should be harvested from compara- 
tive plats of this character. 
It appears that the results from 20 systematically dis- 
tributed quadrates may be fairly safely substituted for the 
yield of the entire plat from which they are taken. 
EXPERIMENTAL ERRORS CAUSED BY SOIL VARIATION 
The lack of uniformly productive land for comparative 
crop tests has given rise to a number of methods frequently 
used for ascertaining and overcoming the resultant experi- 
mental error. Chief among these methods are : (1) The use of 
frequent, systematically distributed check plats planted to a 
uniform crop for the purpose of (a) indicating the degree 
Fig. 13 — A relatively uniform field containing 2 07 thirtieth-acre plats 
sown for a method study to a uniform crop of Kherson oats (1916) 
