document: User-centric content freshness metrics for search engines In order to return relevant search results , a search engine must keep its local repository synchronized to the Web , but it is usually impossible to attain perfect freshness . Hence , it is vital for a production search engine continually to monitor and improve repository freshness . Most previous freshness metrics , formulated in the context of developing better synchronization policies , focused on the web crawler while ignoring other parts of a search engine . But , the freshness of documents in a web crawler does not necessarily translate directly into the freshness of search results as seen by users . We propose metrics for measuring freshness from a user 's perspective , which take into account the latency between when documents are crawled and when they are viewed by users , as well as the variation in user click and view frequency among different documents . We also describe a practical implementation of these metrics that were used in a production search engine .
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