against the background variation shows that the difference between the systems is clearly established in Fig. I a, that Fig. l b suggests strongly that the apparent effect is attributable to random variation, and that in Fig. I c the separation of the two systems is rather more clear than could be established by an analysis for either crop considered alone.


(Q)


    * 0
          0
x x 00 00
         x 0
  xX OX  00

        xx x


(b)


     x0) xx x


(c)


  x   0     0 00
    x
x    x


Figure 1. Different correlation patterns for yields with the same values of the individual crop yields:
(a) negative correlation, (b) positive correlation, (c) no correlation. The two axes are for the yields of the two crops. Two intercrop systems give yields represented by x and o.