Abstract

The ranking aggregation problem is that to establishing a new aggregate ranking given a set of rankings of a finite set of items. This problem is met in various applications, such as the combination of user preferences, the combination of lists of documents retrieved by search engines and the combination of ranked gene lists. In the literature, the ranking aggregation problem has been solved as an optimization of some distance between the rankings overlooking the existence of a true ranking. In this thesis we address the ranking aggregation problem assuming the existence of a true ranking on the set of items: the goal is to estimate an unknown, true ranking given a set of input rankings provided by experts with different approximation quality. We propose a novel solution called Belief Ranking Estimator (BRE) that takes into account two aspects still unexplored in ranking combination: the approximation quality of the experts and for the first time the uncertainty related to each item position in the ranking. BRE estimates in an unsupervised way the true ranking given a set of rankings that are diverse quality estimations of the unknown true ranking. The uncertainty on the items's position in each ranking is modeled within the Belief Function Theory framework, that allows for the combination of subjective knowledge in a non Bayesian way. This innovative application of belief functions to rankings, allows us to encode different sources of a priori knowledge about the correctness of the ranking positions and also to weigh the reliability of the experts involved in the combination. We assessed the performance of our solution on synthetic and real data against state-of-the-art methods. The tests comprise the aggregation of total and partial rankings in different empirical settings aimed at representing the different quality of the input rankings with respect to the true ranking. The results show that BRE provides an effective solution when the input rankings are heterogeneous in terms of approximation quality with respect to the unknown true ranking.