The general approach is to conduct a set of trials
having standard experimental and treatment design, plot
size, and number of replications throughout the region. A
brief description of some of the more important choices to
be made in designing these experiments follows.



Experimental Designs

 A randomized complete block design is preferred,
because of its simplicity and precision. Split-plot or
Latin square designs are also possibilities, but they may
be unnecessarily complex. A separate randomization should
be carried out for each block of the trial at each site.
In other words, a standard randomization should not be
used for all sites.

Number of Replications

 To provide an estimate of the experimental error from
each site, replicates within sites are necessary. Three
or four randomized blocks are recommended, although in
extremely limited land situations two blocks per site
could be used if compensated for by more sites. For
highly variable conditions (such as plant disease control
experiments), more than four replications may be required.

Number of Treatments

 In order to keep the land area small and to limit the
complexity of the trials from a management point of view,
the number of treatments should be as small as possible
and not exceed 15 to 20. As an example, a complete 33
factorial N-P-K rate trial would require 27 treatments,
which would be too many. The number of treatments can be
reduced by such techniques as confounding to generate
incomplete factorials or using other appropriate treatment
designs (double square, central composite, etc.).