Query Difficulty, Robustness and Selective Application of Query Expansion

There is increasing interest in improving the robustness of IR systems, i.e. their effectiveness on difficult queries. A system is robust when it achieves both a high Mean Average Precision (MAP) value for the entire set of topics and a significant MAP value over its worst X topics (MAP(X)). It is a well known fact that Query Expansion (QE) increases global MAP but hurts the performance on the worst topics. A selective application of QE would thus be a natural answer to obtain a more robust retrieval system