where k is one of the j subgroups and P(yi=k) is the probability that
the ith farmer belongs to the k subgroup and xi describes farm and
farmer characteristics. In order to identify the model, constraints must
be imposed. A common approach is to assume that β1=0 (Long,
1997). This normalization makes it possible to identify the coefficients
relative to the base outcome. Applying the constraint, the model can
be written as: