Optimising statistical aspects of sampling designs requires information
about the variances in crop yield that can be expected in both space and
time. The nature of spatial variability must also be known -- do we see any
trends in crop yields across the field or a patchy pattern? If patchy, how big
do the patches tend to be? Do they remain in the same places each season or
not? Some of this information will come from existing on-farm studies, but
often these will not measure crops in sufficient detail to determine what
would have happened if a different measuring scheme had been used. For
this reason it is necessary to carry out some studies aimed specifically at
developing methodology, and one such study is in progress at I C R A F in
Kenya [Pinney, 1990]. Key objectives of this study are not only to understand
the nature of variation in crop yields in farmers fields but to determine how
these can be quickly and easily assessed. The patterns of variation will differ
enormously with cropping system and environment so we need simple ways
of characterising them before starting an on-farm study