Experimental Error in Crop Tests 
53 
USE OF CHECK PLATS 
During the past 15 years it has become the general prac- 
tice in crop investigations to plant check plats at regular 
stated intervals. These plats are planted to a uniform crop 
and should yield alike except for various environmental 
sources of experimental error. 
The use of check plats may be twofold: (1) To indicate 
the error caused by variation in normal plat yields. The 
variation in the check plats is regarded as indicative of the 
error in the test plats. (2) Check plats are more commonly 
used to calculate the normal or theoretical yield of all plats in 
the field. All crops or treatments are then compared directly 
with each other by their increased or decreased yield above 
or below the calculated normal yield for the plats upon which 
they grew. This difference is best expressed in percentage 
of the normal plat yield. Comparative yields per acre may 
then be calculated for each crop, variety, or treatment by 
adding (or subtracting) the difference between it and the 
normal yield for the plat to (or from) the mean yield for all 
check plats in the field. This recalculation of yields is usually 
spoken of as correction according to check plats. 
The check plats may be variously distributed in the field 
according to the manner in which the corrections are to be 
made. Three methods of correction are in common use: (1) 
The normal or theoretical yield of the test plat is determined 
by, and is equivalent to, the average of two adjacent check 
plats. (Alternating plats are check plats.) (2) The normal 
or theoretical yield of the test plat is determined by, and is 
equivalent to, the yield of a single adjacent check plat. (Two 
test plats are planted between checks.) (3) The soil between 
two or more check plats is regarded as varying gradually 
from one check plat to the other and a progressive correction 
is used to establish the normal or theoretical yields of the 
intervening test plats. Thus, if two test plats lie between 
checks which yield 51 and 60 bushels respectively, the nor- 
mal yields assigned to the two test plats by this progressive 
method would be 54 and 57 bushels. Progressing from the 
lower to the higher yielding check the normal yield of the 
first test plat is greater than the poorer check by one-third 
of the difference, while the normal yield of the second test 
plat is greater than the poorer check by two-thirds of the 
difference. The proportion of the difference added to each 
successive test plat will depend upon the number of plats be- 
tween checks. 
