These constraints are then combined by some combination
of intersection (logical AND) or union (logical OR)Boolean constraints. Boolean constraints. Such criteria are typically called factors, and express varying degrees of
suitability for the decision under consideration.
Thus, for example, proximity to roads would be
treated not as an all-or-none buffer zone of suitable
locations, but rather, as a continuous expression of
suitability according to a special numeric scale
(e.g. 0–1, 0–100, 0–255, etc.). The process of
converting data to such numeric scales is most
commonly called standardisation (Voogd 1983).
Traditionally, standardised factors are combined
by means of weighted linear combination – that is,
each factor is multiplied by a weight, with results
being summed to arrive at a multi-criteria solution. In
addition, the result may be multiplied (i.e. intersected)
by the product of any Boolean constraints that may
apply (Eastman et al 1995).