might also consider the magnitude of differences that one
wishes to measure, and then adjust the number of sites so
that the error variance component is small enough to
detect such a difference.

Complexity

 It is desirable that these trials not be highly
complex. This is one reason why use of the randomized
complete block design is recommended. As mentioned above,
treatments may be arranged with either complete or
incomplete factorial structure. Treatments in incomplete
factorials should be chosen in such a way that undue
difficulty in analysis is avoided. Every attempt should
be made to standardize the number of blocks, the
treatments, and the plot and block dimensions, and to
avoid missing values for any of the plots. Trials that
differ from the others with respect to these
considerations could complicate the analysis. All
management operations should be recorded at each site
for use in interpreting results.

Replications over Years

 Multi-year testing to ascertain stability of results
occurs in the farming systems approach as alternative
technologies move through the sequence of researcher- and
farmer-managed trials. Evaluating technologies over a
range of environments also aids in the evaluation of
stability. For this reason, it is not normally necessary
to repeat the same trial for two or more years, as is
usual on experiment stations, which represent only a
single site.

COMBINING DATA OVER SITES

 One of the goals in regional trials is to provide an
estimate of interaction of sites (environments) and
treatments. One way this can be accomplished is with a
combined analysis of variance over sites. If this
interaction is negligible, estimates of the treatment
effects over sites, which would be used for
interpretation, would be stable. This would imply that
the recommendation domain is homogeneous with respect to