Why do you think this query is difficult? A user study on human query prediction

Predicting if a query will be difficult for a system is important to improve retrieval effectiveness by implementing specific processing. There have been several attempts to predict difficulty, both automatically and manually; but without high accuracy at a preretrieval stage. In this paper, we focus rather on understanding why a query is perceived by humans as difficult. We ran two separated but related experiments in which we asked humans to provide both a query difficulty prediction and reasons to explain their prediction. Results show that: (i) reasons can be categorized into 4 classes; (ii) reasons can be framed into closed questions to be answered on a Likert scale; and (iii) some reasons correlate in a coherent way with the human predicted numerical difficulty. On the basis of these results it is possible to derive hints to be provided to help users when formulating their queries and to avoid them to rely on their wrong perception of difficulty.