Résumé en anglais

Some multicriteria decision models allow decision-makers to express that two alternatives are incomparable: i.e. no strict preference nor indifference holds. Various motives can lead decision- makers to express incomparability: uncertainty on evaluations of the alternatives, unsufficient ex- perience in the decision field, refusal to endorse a commiting decision. . . This research investigates incompleteness of preferences resulting from multicriteria conflict. In a conflictual comparison, each alternative is superior to the other on at least one attribute (i.e. no dominance holds). In this work, we investigate empirically the effect of conflict between alternatives and proximity of the isopreference curves to which they belong on the tendency to express incomparability. We test the hypothesis that comparisons involving alternatives located on the same isopreference curve, and with a high conflict are more likely to elicit incomparability statements. We also test the confused symmetry hypothesis following which when assessed by elicitation technique allowing indifference as the only symmetrical answer, decision-makers will use it to express indifference or incomparability, indiscriminately. We carried out a behavioral experiment on 47 undergraduate students at Ecole Centrale Paris, who were likely to be looking for an accomodation for their upcoming training course. Their main task was to make pairwise comparisons on apartments which differed on rent and accessibility. We found that a greater conflict does lead to a greater number of incomparability statements, but also that proximity of isopreference curves does not increase incomparability. Our data strongly confirms the confused symmetry hypothesis, and we propose a heuristics model following which decision- makers first choose to express a strict or symmetrical preference, then determine which symmetrical preference fits their “views” or their thoughts better (if indifference and incomparability are both allowed). Moreover, we found that for comparisons involving alternatives located on the same isopreference curve, incomparability decreases with their overall value.