Document Ranking for Effectiveness-Efficiency Tradeoffs

Method: Building a document ranking system involves two key decisions: choosing a retrieval model, and choosing a suitable index representation. The former determines the effectiveness of the system, the latter the efficiency; and each of them affects the other. The impact-based document ranking mechanism described by Anh and Moffat [2] was chosen for our system because of its balance between effectiveness and efficiency. In terms of effectiveness, it is highly competitive, although still inferior to advanced language modelling implementations. On the other hand, in terms of efficiency the mechanism is excellent, as it ranks documents using a small number of calculations, all on integer numbers. To further facilitate query efficiency, we compress the index using the slide-8 coding scheme [1], which allows an excellent balance between compression ratio and decoding speed.