fertilizer?) On the other hand, variety produces a
significant yield increase and its effect on cost is
minimal, making it a good alternative to introduce in the
production system in which the factors were evaluated. If
the plant density change does not require too much additional
labor at planting time, this may also be a good
alternative to introduce.

Comments on Reducing the Size of the Trial

 Generally, many degrees of freedom in this type of
design (2n) are associated with higher-order interactions
which are difficult to interpret. If the higher-order
interactions (third order and higher) are not considered,
the size of the trial would be substantially reduced,
keeping some of the advantages of the basic factorial
arrangements. In this case, it is advantageous to use the
fractional factorial (Cochran and Cox, 1957). Example:
with n = 8, main effects and first-order interactions can
be estimated with only (1/8) x 28 = 32 treatments.
Equally, main effects and second-order interactions can be
estimated with only (1/4) x 28 = 64 treatments.
 Another way to reduce the number of treatments in a
2n factorial is to select factors based on their
importance. Factors and combinations which are considered
of little interest from the biological and economic
viewpoint, or those which do not interact, can be
eliminated. For instance, if selecting two factors, A and
B, the following treatments can be established, with two
levels each: Al Bl, A2 Bl, Al B2, and A2 B2. If there is
no interaction between the two factors, determined from
previous experimental information, the main effect of A
corresponds to the average difference between A2 Bl and Al
Bl, and the effect of B to the average difference between
Al B2 and Al Bl.



THE "PLUS" TRIAL

 Exploratory information on new variables as they
relate to existing practices can be obtained by testing,
one at a time, a series of alternatives that include the
new variables. The following example compares a
traditional maize practice with three alternatives. It