I just read the recent Information Processing & Management special issue on Evaluation of Interactive Information Retrieval Systems. The articles were a worthwhile read, and yet they weren’t exactly what I was looking for. Let me explain.
In fact, let’s start by going back to Cranfield. The Cranfield paradigm offers us a quantitative, repeatable means to evaluate information retrieval systems. Its proponents make a strong case that it is effective and cost-effective. Its critics object that it measures the wrong thing because it neglects the user.
But let’s look a bit harder at the proponents’ case. The primary measure in use today is average precision–indeed, most authors of SIGIR papers validate their proposed approaches by demonstrating increased mean average precision (MAP) over a standard test collection of queries. The dominance of average precision as a measure is no accident: it has been shown to be the best single predictor of the precision-recall graph.
So why are folks like me complaining? There are the various user studies asserting that MAP does not predict user performance on search tasks. Those have me at hello, but the studies are controversial in the information retrieval community, and in any case not constructive.
Traditionally, information retrieval systems aim to maximize the number of relevant documents returned to a user within some window of the top. For that goal, the probability ranking principle, which ranks documents in decreasing order of probability of relevance, is provably optimal. However, there are many scenarios in which that ranking does not optimize for the user’s information need.
Let me rephrase that: the precision-recall graph, which indicates how well a ranked retrieval algorithms does at ranking relevant documents ahead of irrelevant ones, does not necessarily characterize how well a system meets a user’s information need.
One of Chen and Karger’s examples is the case where the user is only interested in retrieving one relevant document. In this case, a system does well to return a diverse set of results that hedges against different possible query interpretations or query processing strategies. The authors also discuss more general scenarios, along with heuristics to address them.
But the main contribution of this paper, at least in my eyes, is a philosophical one. The authors consider the diversity of user needs and offer quantitative, repeatable way to evaluate information retrieval systems with respect to different needs. Granted, they do not even consider the challenge of evaluating interactive information retrieval. But they do set a good example.
Stay tuned for more musings on this theme…