The interpretation indicated by the analysis and mean yields is as follows. Yield of cowpea is substantially determined by the maize variety grown with the cowpea. Higher cowpea yields are obtained when maize variety I is grown. For cowpea variety B, cowpea yield is reduced as increasing amounts of nitrogen are applied (presumably because of correspondingly improved maize yield). Yields for cowpea variety A are not affected in this manner.

2.2 Assumptions in the Analysis of Variance

The interpretation of an analysis of variance and of the subsequent comparisons of treatment means depends critically on the correctness of three assumptions made in the course of the analysis. If the assumptions are not valid, the conclusions drawn may also be invalid and, therefore, misleading. Evidence available from the analyses of intercropping experiments suggests that failure of the assumptions is at least not less frequent than in monoculture experiments. It is therefore vital that the experimenter deliberately consider the assumptions before completing the analysis. The three assumptions are:

1    That the variability of results does not vary between treatments
2    That treatment differences are consistent over blocks
3    That observations for any particular treatment for units within a single block would be approximately
     normally distributed

Table 1. Cowpea and Maize Yields in intercrop Trial at 11TA, Nigeria


                                                   Yield (kg/ha)a
                                      1                         2                     3
    Cowpea       Nitrogen        ________               _________                _______variety            level         1     I1       I11     1        1       III     I     H       II


                                                         Cowpea
        A           No         259     645      470     523     540     380      585   455     484
        A           NI         614     470      753     408     321     448     427    305     387
        A           N2         355     570      435     311     457     435     361    586     208
        A           N3         609     837      671     459     483     447     416    357     324
        B           No         601     707      879     403     308     715     590    490     676
        B           Nj         627     470      657     351     469     602     527    321     447
        B           Ni         608     590      765     425     262     6 12    259    263     526
        B           N3         369     499      506     272     4211    280     304    295     357

                                                          Maize
        A           No        2121    2675    3 162    22 54   3628 4069      2395  2975    4576
        A           Ni1       3055    3262    3749     3989    3989 4429      4429 4135     4429
        A           N2        3922    3955    4095     4642    4135 4642      5589 4429     5156
        A           N3        4129    4129    4 022    3975    4789 4282      5990  5336    5663
        B           No        2535    2535    2288     4209    3989 2321      2901 4429     3482
        B           Ni1       2675    3402    3122     4789    4936 3342      3555 4936     4135
        B           N2        3855    3815    3535     5083    4496  3702     60231 5296    4069
        B           N3        3815    4202    3749     5656    5516  5223     5516 5083     5369


aYields grouped by maize variety (1, 2, 3) and planting block (1, 11, 111). Source: Data from Dr. Ezumab, UTA, unpublished.