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REVIEW OF INTERCROPPING ANALYSIS METHODOLOGY

1. Measurements and Analysis

The first point to recognise is that there is not a single form of statistical analysis which is appropriate to all forms of intercropping data. Even for a single set of experimental data it will be important to use several different forms of analysis. For the two components of an intercropping system the data may occur in different structural forms. In general, data structures from intercropping experiments will be complex with different forms of yield information available for different subsets of experimental units.

1.1 Valid Comparisons

In considering alternative possibilities for the analysis of data from intercropping experiments it is essential that the principle of comparing "like with like" is obeyed. If yields are measured in different units, or over different time periods, or for different species, then in general comparisons will not be valid and should not be attempted. To illustrate the difficulties and possibilities we consider a set of ten "treatments". Any actual experiment would be unlikely to include such a diverse set of treatments though there would typically be several representatives of some of the "treatment types" illustrated. The structure for the ten treatments is as follows.


           Legume Crop               Cereal Crop           Monetary             Relative
        Species     Yield        Species      Yield          Value            Performance


  I)        I        y                                        r2)          II        Z2                                       r             
  3)                                A           a3             r3
  4)        -                       B           b4             r4
  5)        I        Y5             A           a5             r5            ys/yI + a5/a3
  6)        I        Y6             A           a6             r6            y6/y I + a6/a3
  7)        I        y7             B           b7             r7            y71Y I + b7/b4
  8)        I        y8             B           b8             rg            y8/y I + bs/b4
  9)        II        z9            A           a9             r9            zg/z2 + aq/a3
10)         11        zlo           B           blo            rio           zIo/z2+blo/b4


A comparison is valid only when the units of measurement are identical. Thus it is valid to investigate the effect of different cereal crops on legume yields of one species (y 1, ys, y6. y7. ygx or of the other species (z2, z9, zmo). Similarly the effect of different legume environments on crop yield (a3, a5, a6, a9) or (b4, b7, bs, bIo). The effects of different treatment systems on pairs of yields may be assessed by comparing the pair (Y5, a5) with (Y6, a6) or (y7, b7) with (y8, b8). Particular combinations of the pair of yields may also be compared so that (y5/yj + as/a3) may be compared (y6/yl + a6/a3). However it is not valid to compare