been adopted by the farmers within the region, so this
factor will not need to be included within the trial. The
ranges in rates to be used are 0 to 100 kg/ha for N and 0
to 120 kg/ha for P.

 N
 (+2,0)

 (+1,-1) (+1,+i)

 (0,-2) (0,0) (0,2)

 (-1,-1) (-1,+1)

 (-2,0)
 P

The coding scale is as follows:

Actual N: 0 25 50 75 100 Actual P: 0 30 60 90 120
Coded N:-2 -1 0 +1 +2 Coded P:-2 -1 0 +1 +2

FIG. V-l. Modified central composite design.

 The design selected is a modification of the central
composite response surface (described by Cochran and Cox,
1957).

Selection of Sites

 Sites should normally be selected to cover a range in
the environmental characteristics whose interaction with
the technologies to be tested is considered to be of
interest. These environmental characteristics could
include soil nutrient levels, soil moisture, climatic
effects, management, etc. The number of sites required
will depend upon the variability in the region and how the
results will be used (that is, the analysis or analyses to
be employed). For example, if modified stability analysis
is to be used where an environmental index is estimated,
as suggested in Chapter VII, a minimum of eight to ten
sites (perhaps as few as five) is necessary. This may
also be a reasonable number for estimating the component
of variation due to sites by analysis of variance. One