of one variable (yield), after the effect of a possible
disturbing variable (incomplete plant stands) has been
removed. The application of covariance must be justified
in the sense that the correction is being made because of
an uncontrolled environmental condition that does not
affect all the observations (plots) in the trial in a
homogeneously constant fashion. The example to be
presented in this section is a correction of yield in a
trial where animals ate plants in different plots, more in
some than in others. This exogeneous effect is not
related to the ability of the cultivars under evaluation
to withstand adverse environments. However, if grazing is
part of the environment in which the crop will be
produced, corrections should not be made.
 The example presented is an evaluation of six new
maize hybrids in a randomized complete block design with
four replications.

Step 1. Table II-2 shows the of number of plants (X) and
kilograms per plot (Y) for each cultivar.

TABLE 11-2. Number of plants (X) and kilograms per
 plot (Y) of six maize hybrids.

Treat- BLOC K S
ment I II III IV TOTAL
No. X Y X Y X Y X Y X Y
1 60 3.42 59 5.25 62 4.52 60 6.24 241 19.43
2 47 2.87 60 3.97 61 3.12 51 4.82 219 14.78
3 51 4.21 35 3.29 60 5.58 40 3.82 186 16.90
4 58 2.58 32 1.93 62 3.07 50 4.05 202 11.63
5 62 3.28 54 4.13 60 4.05 62 5.38 238 16.84
6 40 1.98 62 5.02 61 3.89 61 5.20 224 16.09

TOTALS 318 18.34 302 23.59 366 24.23 324 29.51 1310 95.67

Source: ICTA, Guatemala