The oldest reference we have found dates from 1972 (Saaty, 1972). Problem modelling As with all decision-making processes, the facilitator will sit a long time with the decision-maker(s) to structure the problem, which can be divided into three parts: goal (buy a car), criteria (initial cost, maintenance cost, prestige, quality and its sub-criteria) and alternatives (Fiat Uno, Nissan Maxima 4 Doors, Mercedes Benz 290, Volvo 840, Ford Fiesta) (Figure 1). AHP has the advantage of permitting a hierarchical structure of the criteria, which provides users with a better focus on specific criteria and sub-criteria when allocating the weights. Pairwise comparisons At each node of the hierarchy, a matrix will collect the pairwise comparisons of the decision-maker (for example Figure 2). Psychologists argue that it is easier and more accurate to express one’s opinion on only two alternatives than simultaneously on all the alternatives. It also allows consistency and cross checking between the different pairwise comparisons (see Consistency part under the section The original AHP method). AHP uses a ratio scale, which, contrary to methods using interval scales (Kainulainen et al, 2009), requires no units in the comparison. The judgement is a relative value or a quotient a/b of two quantities a and b having the same units (intensity, meters, utility and so on). Their RIs are different but close to Saaty’s.
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