To fully realize the ambition of this metric, we would directly identify a sample of required end-task facts, e.g., by manual analysis of questions posed to the end-task system, or from logs of the interaction between the endtask system and the KB.
given the practical challenges of doing this at scale, we take a simpler approach and approximate this end-task distribution using facts extracted from an (independent) domain-specific text corpus (we call this a reference corpus).