or other means, and individual treatments are installed
either by the researcher or the farmer. Together, the
researcher and the farmer harvest the crop when it is
mature. The design of a superimposed trial should be
simple. Replications should be used at each location,
although data from designs without replications at each
site can be combined for regional analysis and
interpretation.
 An example of a simple superimposed trial from IRRI
is shown in Tables III-1 and III-2. Previous information
indicated that rice responded to at least 50 kg/ha of
nitrogen, but response to potassium and phosphorus was
uncertain. A simple six-treatment superimposed trial was
established on a number of farms and information was
obtained from six of them (Table III-1). The design at
each location was without replication. The six treatments
included three levels of nitrogen (50, 70, and 90 kg/ha).
At 70 kg/ha of N the treatments explored the application
of 30 kg/ha phosphorus and potassium individually and
together. Analysis of variance (Table III-2) indicated a
significant effect for nitrogen but none for the other
elements. The conclusion was that more nitrogen would
have a positive effect on yield, and if the cost were less
than the value of the additional crop, more nitrogen
could be recommended. Furthermore, it would indicate that
additional work should be done with nitrogen, but the
other major elements (P and K) need not be studied further
in this context.

TABLE III-2. Analysis of variance of a superimposed
 N-P-K rice trial.

Source d.f. SS MS Fc

Total 35 69,071
Farms 5 32,178
Treatments 5 11,212 2,242
 N 2 9,837 4,918 4.79
 Remainder 3 1,375 458 0.45
Error 25 25,681 1,027
CV = 7.6%
Source: Zandstra et al. (1981), p. 107