following situation: under traditional practices, 20
workdays are required to weed a hectare of land that
produces 3200 kg of product. A new arrangement in
planting density and spacing adds 20% to labor
requirements for weeding and results in 500 kg of
additional production. Traditional practices result in
160 kg per workday spent in weeding (3200 / 20) but the
alternative technology yields 3700 kg for 24 workdays or
154.2 kg per workday. Because labor for weeding is
limiting, the relevant evaluation criterion is not yield
per hectare (3700 vs. 3200 kg). If farmers are limited to
30 workdays of labor in the weeding period, during those
30 workdays they are able to weed less land in the new
system. Because each workday results in 154.2 kg of
product with the new technology, rather than 160 kg with
the traditional technology, the 30 workdays produce only
4626 kg under the alternative technology, compared with
4800 kg under the traditional technology. In this
example, it is evident that the criterion of yield per
hectare (3700 kg compared with 3200 kg) results in a false
conclusion regarding the value of the technology to the
farmers. With a limit of 30 workdays of labor available
for weeding, this leaves farmers with a choice of 4800 kg
of product with the traditional system or 4626 kg using
the new technology.
 Measures of productivity of labor (kg/workday) are
subject to the same kind of variation as other measures of
productivity, such as kg/ha. A common mistake in making
an economic analysis is to make only one estimate and
assume it is firm. Measures of productivity (evaluation
criteria) can be subjected to the same kinds of
statistical analysis as are commonly used for biological
or agronomic criteria. For example, net income can be
affected by variation in yield, price of the product, use
of inputs, and price of inputs. When farmers' practices
and prices paid or received vary, separate income
calculations should be made for each farm, just as
separate yields are measured for each farm.