
Xiaoye Jiang, LekHeng Lim, Yuan Yao, and Yinyu Ye. Learning to Rank with Combinatorial Hodge Theory. Mathematical Programming, Series B, 127:203244, 2011.
Abstract:
We propose a number of techniques for learning a global ranking from data that may be incomplete and imbalanced characteristics that are almost universal to modern datasets coming from ecommerce and internet applications. We are primarily interested in cardinal data based on scores or ratings though our methods also give specific insights on ordinal data. From raw ranking data, we construct pairwise rankings, represented as edge flows on an appropriate graph. Our rank learning method exploits the graph Helmholtzian, which is the graph theoretic analogue of the Helmholtz operator or vector Laplacian, in much the same way the graph Laplacian is an analogue of the Laplace operator or scalar Laplacian. We shall study the graph Helmholtzian using combinatorial Hodge theory, which provides a way to unravel ranking information from edge flows. In particular, we show that every edge flow representing pairwise ranking can be resolved into two orthogonal components, a gradient flow that represents the l2 optimal global ranking and a divergencefree flow (cyclic) that measures the validity of the global ranking obtained — if this is large, then it indicates that the data does not have a good global ranking. This divergencefree flow can be further decomposed orthogonally into a curl flow (locally cyclic) and a harmonic flow (locally acyclic but globally cyclic); these provides information on whether inconsistency in the ranking data arises locally or globally.
When applied to the problem of rank learning, Hodge decomposition sheds light on whether a given dataset may be globally ranked in a meaningful way or if the data is inherently inconsistent and thus could not have any reasonable global ranking; in the latter case it provides information on the nature of the inconsistencies. An obvious advantage over the NPhardness of Kemeny optimization is that the discrete Hodge decomposition may be easily computed via a linear least squares regression. We also investigated the l1 projection of edge flows, showing that this has a dual given by correlation maximization over bounded divergencefree flows, and the l1 approximate sparse cyclic ranking, showing that this has a dual given by correlation maximization over bounded curlfree flows. We discuss connections with wellknown ordinal ranking techniques such as Kemeny optimization and Borda count from social choice theory.
Bibtex:
@article{jlyylrcht09,
author={Xiaoye Jiang and LekHeng Lim and Yuan Yao and Yinyu Ye},
title={statistical ranking and combinatorial hodge theory},
journal={mathematical programming, series B},
volume={127},
pages={203244},
year={2011}
}

