CHOICE OF EVALUATION CRITERIA


 One of the most important and critical decisions to
be made when designing research and evaluating data is the
selection of choice criteria for evaluation. The most
common evaluation criterion used by agronomists is yield
per unit of land area, frequently kg/ha. The use of this
criterion implies that land is the most limiting resource
on the farm and that therefore productivity of the land is
the most important evaluation criterion. This is not
always the case. On many small farms, even though there
is little land, land is not the most limiting constraint.
Nor is the same constraint necessarily the most limiting
for different crops. For example, small farmers in
Nariio, in the south of Colombia, traditionally plant
their scarce potato seed by spacing it widely to maximize
the productivity per unit of potato seed. The amount of
seed determines the size of the potato field. Hence, land
is not the most limiting resource with respect to potato
production on these small farms. However, the rest of the
land on these farms is planted into grain crops. For
grain, land is a limiting resource. For this reason, in
the case of potatoes, technological changes which increase
the productivity per unit of land area but decrease the
productivity per unit of seed will not be attractive to
these farmers. On the other hand, the same kind of
technology for grain crops could be acceptable. The
importance of using the relevant choice criterion in
evaluating on-farm trials is obvious in this case.
 In large areas of Africa, land is not a limiting
resource. Farmers can plant as much land as they are able
to manage. However, in these same areas, rainfall is
scarce, so weeding the crops becomes a critical factor.
These farmers tend to plant the amount of land they can
effectively weed, because planting more land is a waste of
effort if it cannot be weeded. In this case, labor for
weeding becomes the most important evaluation criterion
and changes in crop production practices must be evaluated
against this factor.
 In some areas, such as eastern Guatemala, crops must
be planted as soon as possible after the initiation of the
rains. Delayed planting reduces yield heavily because of
a mid-season dry spell, increased pest problems, or
because the crop does not mature before the rains