CLEF 2007: ad hoc track overview

We describe the objectives and organization of the CLEF 2007 Ad Hoc track and discuss the main characteristics of the tasks offered to test monolingual and cross-language textual document retrieval systems. The track was divided into two streams. The main stream offered mono- and bilingual tasks on target collections for central European languages (Bulgarian, Czech and Hungarian). Similarly to last year, a bilingual task that encouraged system testing with non-European languages against English documents was also offered; this year, particular attention was given to Indian languages. The second stream, designed for more experienced participants, offered mono- and bilingual "robust" tasks with the objective of privileging experiments which achieve good stable performance over all queries rather than high average performance. These experiments re-used CLEF test collections from previous years in three languages (English, French, and Portuguese). The performance achieved for each task is presented and discussed.