Hayes and Garber - Cucurbit Breeding

52 BREEDING CROP PLANTS
certain contiguous units tend to yield high while others show a
tendency in the opposite direction. Under these conditions a
high correlation coefficient results. If variability due to random
sampling only is entering, the correspondence between some
contiguous plots will be counterbalanced by the lack of correspondence
between others, providing that the number of ultimate
units is sufficiently large to permit an expression of the law of
average. It is obvious that in the application of Harris' method
the field must receive the same treatment (seed, cultivation,
fertilizer, etc.). The division of the field into the desired units
may be made at any time before the crop is harvested, but
preferably before or soon after planting in order to minimize
possible injury to the growing crop.
A simple illustration will make the calculation of the correlation
coefficient clear, although a much larger number of units
should be used in an actual study of the reliability
of a field for
plot work. Suppose a certain field is divided into 16 units and
these units are in turn arranged in groups. Let pi, p 2 , p3 , etc.,
represent the ultimate units and C Pv C PV etc., represent the
groups.
By assigning values for yield in bushels per acre to the
ultimate units, one may make the calculation necessary to apply
the formula. The value of any particular group is the sum of
the ultimate units in it.
DIAGRAM ILLUSTRATING HARRIS'
METHOD
(2)