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The results of a given query are sorted by relevance score. Relevance score is computed based on some factors like term frequency, document frequency, where the querying terms matched in the document (body, title, summary...). The downside of sorting purely by relevance can be ignores the important of up-to-date content such as news articles. See Combined Time and Relevance Sorting for more details.
How to Configure
This setting can be defined either using your browser, when you create a new project, using the Advanced Options or by changing the value of time_relevance in common.ini, in the [ranking] section.