 One important companion calculation is called “complementarity” (Kirkpatrick 1983)—the gains and losses in biodiversity as objects are gained or lost. While biodiversity is quantified by an inferred count of number of different units, decision-making about biodiversity uses various calculations based on those units. Complementarity usefully indicates marginal changes—the number of units lost, or the increase in the number represented by an added protected area. Faith (1994) focussed on complementarity as an example of useful calculations (referred to as “components of biodiversity”) based on inferred counts of units: