Where to stop reading a ranked list?: threshold optimization using truncated score distributions

  • Authors:
  • Avi Arampatzis;Jaap Kamps;Stephen Robertson

  • Affiliations:
  • University of Amsterdam, Amsterdam, Netherlands;University of Amsterdam, Amsterdam, Netherlands;Microsoft Research, Cambridge, United Kingdom

  • Venue:
  • Proceedings of the 32nd international ACM SIGIR conference on Research and development in information retrieval
  • Year:
  • 2009

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Abstract

Ranked retrieval has a particular disadvantage in comparison with traditional Boolean retrieval: there is no clear cut-off point where to stop consulting results. This is a serious problem in some setups. We investigate and further develop methods to select the rank cut-off value which optimizes a given effectiveness measure. Assuming no other input than a system's output for a query--document scores and their distribution--the task is essentially a score-distributional threshold optimization problem. The recent trend in modeling score distributions is to use a normal-exponential mixture: normal for relevant, and exponential for non-relevant document scores. We discuss the two main theoretical problems with the current model, support incompatibility and non-convexity, and develop new models that address them. The main contributions of the paper are two truncated normal-exponential models, varying in the way the out-truncated score ranges are handled. We conduct a range of experiments using the TREC 2007 and 2008 Legal Track data, and show that the truncated models lead to significantly better results.