Promoting Relevant Results in Time-Ranked Mail Search

Mail search has traditionally served time-ranked results, even if it has been shown that relevance ranking provides higher retrieval quality on average. Some Web mail services have recently started to provide relevance ranking options such as the relevance toggle in the search results page of Yahoo Mail, or the ``top results" section in Inbox by Gmail. Yet, ranking results by relevance is not accepted by all, either in mail search, or in in other domains such as social media, where it has even triggered some public outcry. Given the sensitivity of the topic, we propose here to investigate a mixed approach of promoting the most relevant results, to which we refer as ``heroes'', on top of time-ranked results. We argue that this approach represents a good compromise to mail searchers, supporting on one hand the time sorted paradigm they are familiar with, while being almost as effective as full relevance ranking view that Web mail users seem to be reluctant to adopt. We describe three hero-selection algorithms we have devised and the associated experiments we have conducted in Yahoo mail. We measure retrieval success via two metrics: MRR (Mean Reciprocal Rank) and Success@k, and verify agreement between these metrics and users' direct feedback. We demonstrate that supplementing time-sorted results with hero results leads to a higher MRR than the traditional time-sorted view. We additionally show that MRR better reflects users' perception of quality than Success@k. Finally, we report on online results following the successful launch of one of our hero-selection algorithms for all Yahoo enterprise mail users and a few million Yahoo Web mail users.