186 
JOURNAL OF ECONOMIC ENTOMOLOGY 
[Vol. 17 
field plats is one of the causes of excessive variation and a powerful 
factor in producing uncertainty in the results of a single set of experi¬ 
ments. Generally, conditions of homogeneity are approximated, to a 
greater or less extent, by the astuteness of the investigator in observing 
conditions previous to making the experiments. As will be described 
subsequently, methods of measuring the degree of heterogeneity in 
field plats have been developed and should be used wherever possible. 
Two general methods of compensating for heterogeneity have been in 
common use and are still to be recommended: (1) duplication and re¬ 
versal of plats of uniform size in the same orchard, taking a mean of 
plats treated alike; and (2) the use of a number of plantings in which the 
tests are repeated, either the same or other seasons. 
Determination of Heterogeneity 
With varieties that bear annually, one of the safest methods of de¬ 
termining lack of homogeneity would be to select the orchard the year 
previous to starting the tests and to make counts of infested apples. 
Having a sufficient number of adjacent trees, the data can be analyzed 
first for systematic error and then for general heterogenity. 
Systematic Error. If the values determined thruout an area tend to 
increase or decrease from one side to another, or, if certain areas have 
values considerably higher or lower than others, the material is affected 
by systematic, error. When this is quite uniform across the orchard, 
corrections can be computed and applied or plats can be duplicated in a 
manner to compensate for the differences. 
Heterogeneity. The most common lack of uniformity is from tree to 
tree or between groups of trees. To measure this form of heterogeneity, 
the methods and formulae of Harris 1 are the best known to the writer 
Wherever possible there criteria should be applied. The fact that they 
have been so well described and since the publication is available, further 
description is unnecessary. 
Variation Due to Casual and Undetermined Factors 
Variation or individuality is associated with all living matter. This 
tendency for individuals to vary is responsible for much of the difficulty 
in the interpretation of the data of field tests, even tho the greatest 
efforts are made to select homogeneous material and avoide rrors. The 
Harris, J. Arthur. Practical Universality of Field Heterogeneity as a Factor 
Influencing Plat Yields. Tour. Arg. Research 19:.pp 279-314. 1920. 
