What specific type of retrieval failure is primarily mitigated by employing 'multi-query retrieval' techniques?

The specific type of retrieval failure primarily mitigated by employing 'multi-query retrieval' techniques is recall failure stemming from an insufficient or misspecified single query. Retrieval failure occurs when a search system does not return the desired or expected set of relevant documents for a given information need. Recall failure, specifically, is a type of retrieval failure where the system fails to retrieve a significant portion of the relevant documents available in the corpus, meaning many relevant items are missed. This particular recall failure arises when a single, initial search query is either too narrow, uses non-standard terminology, or fails to capture the full, multifaceted nature of the user's underlying information need. This inability of a single query to fully represent the user's intent or cover all relevant aspects is known as query misspecification or insufficient query formulation. Multi-query retrieval is a technique that addresses this by generating and executing multiple distinct but related search queries based on an initial user request. Instead of relying on one potentially suboptimal query, multi-query retrieval creates several variations or expansions of the original query, each designed to explore different facets, synonyms, or reformulations of the user's information need. By casting a wider net with these multiple queries and then combining their results, the system significantly increases the likelihood of capturing relevant documents that a single, perhaps too specific, ambiguous, or incompletely phrased, query might have otherwise overlooked. This process directly combats the problem of under-retrieval caused by the inherent limitations of a solitary query, thereby improving overall recall.