comparing two mean yield pairs equal in all directions. The results in Fig. 2 are for the three maize varieties from the example, and the size of the standard error of a difference between two mean pairs is shown by the radius of the circle.




                              Maize Varieties


70OO


5000


Cowpea


Maize


Figure 2. Bivariate plot of pairs of mean yields for three maize varieties (1,2.3). Maize and cowpea yields are in kilograms per hectare. (Data from Table 1).

Construction of the skew axes diagram is based on the original papers of Pearce and Gilliver (1978, 1979) and detailed instructions for construction are given by Dear and Mead (1983, 1984). The form of the diagram given in Fig. 2 treats the two crops symmetrically, in contrast to the original suggestion of Pearce and Gilliver, in which one yield axis is vertical and the other is diagonally above or below the horizontal axis, depending on the sign of the error correlation. A summary of the method for construction of the symmetric diagram is as follows:

     If the error mean squares for the two crops are VI (= 0.346 in the example) and V2 (= 0.0130), and
     the covariance is V12 (= -0.0310), then the angle between the axes 0 is defined by

                                       Cos 0 =    V2
                                                (V1V2)":

     If the range of values for the two yields XI and X2 are (Xo, Xi ) and (X2o, X2) respectively, then
     we define two new variables y1 and y1,
                                           x1
                                    Y1 = x = Kjx2


x2 - V12x/V1=2,      (      Lx
2 - V(,/V,)"2         x2        )