1. Introduction: the robustness problem of information retrieval -- Reasons for retrieval failures, the RIA workshop -- Instability in retrieval, the TREC's robust tracks -- Estimating the query difficulty -- Summary
2. Basic concepts -- The retrieval task -- The prediction task -- Linear correlation -- Rank correlation -- Prediction robustness -- Summary
4. Pre-retrieval prediction methods -- Linguistic approaches -- Statistical approaches -- Definitions -- Specificity -- Similarity -- Coherency -- Term relatedness -- Evaluating pre-retrieval methods -- Summary
5. Post-retrieval prediction methods -- Clarity -- Definition -- Examples -- Other clarity measures -- Robustness -- Query perturbation -- Document perturbation -- Retrieval perturbation -- Cohesion -- Score distribution analysis -- Evaluating post-retrieval methods -- Prediction sensitivity -- Summary
6. Combining predictors -- Linear regression -- Combining pre-retrieval predictors -- Combining post-retrieval predictors -- Combining predictors based on statistical decision theory -- Evaluating the UEF framework -- Results -- Combining predictors in the UEF model -- Summary
7. A general model for query difficulty -- Geometrical illustration -- General model -- Validating the general model -- The relationship between aspect coverage and query difficulty -- Validating the relationship between aspect coverage and query difficulty -- Summary
8. Applications of query difficulty estimation -- Feedback: to the user and to the system -- Federation and metasearch -- Content enhancement using missing content analysis -- Selective query expansion -- Selective expansion based on query drift estimation -- Adaptive use of pseudo relevance feedback -- Other uses of query difficulty prediction -- Summary
9. Summary and conclusions -- Summary -- What next -- Concluding remarks -- Bibliography -- Authors' biographies.