1. A method of computing an aggregate decision about an object, the method comprising: obtaining at least two intermediate rankings, each of which ranks at least three alternatives with respect to the object; initializing at least two series of preferences, wherein each series corresponds to one of the intermediate rankings, each series is ordered from a first preference to a last preference, each series initially groups each alternative into one of the preferences selected based on the corresponding intermediate ranking, and each preference includes at least one of the alternatives; selecting, based on the preferences, at least one of the alternatives as a weak alternative; removing at least one weak alternative from each series; then re-grouping each series into preferences based on the intermediate ranking corresponding to the series; deciding, when each series contains less than a first threshold number of the alternatives, that the aggregate decision includes the alternatives remaining in the series; and repeating selecting, removing, re-grouping, and deciding until an aggregate decision is made.