where Y is yield and e is the environmental index for
each farm. The equations as estimated by least squares
are summarized in Table VII-8. The equations are shown
graphically in Fig. VII-10.

TABLE VII-8. Linear equations for modified stability
 analysis calculated from radish data.

 Treatment
 cwt N/acre a b R2



 0 0.13 0.54 0.55
 1 0.04 1.24 0.81
 2 0.31 0.93 0.80
 3 -0.23 1.21 0.84
 4 -0.26 1.08 0.76



 To obtain the data for the response surfaces,
estimates of yield for each N level can be taken from the
graph, or the linear equations in Table VII-8 can be
solved. The latter is more precise. Yields are shown in
Table VII-9 for two environmental levels, e = 1.5 and e =
2.5, which lie within the range of data. These values
were obtained by solving the equations in Table VII-8 for
the two values of e and for each N level.


TABLE VII-9.


Estimated radish yield for five
levels of N for two environments.


 cwt N/acre
 e 0 1 2 3 4
 tons/acre

 1.5 0.94 1.90 1.70 1.58 1.36
 2.5 1.48 3.14 2.64 2.80 2.44



 The data in Table VII-9 are then used to obtain two
response surfaces. A quadratic surface seems appropriate,
based on the type of response. The two equations,
estimated by least squares, follow: