20. The media of claim 19 , further comprising instructions that when executed by the at least one processor cause the at least one processor to: identify a relevance of a content item retrieved using the topic to perform a per interest retrieval to be the relevance value reflecting the strength of relationship between the topic and the content item; and identify a relevance of a content item retrieved in a group interest retrieval using the remaining topics, the identified relevance being identified using a cosine similarity between the remaining topics' relevance values, each relevance value corresponding to one of the remaining topics and reflecting the strength of relationship between the remaining topic and the content item.
19. The media of claim 18 , further comprising instructions that when executed by the at least one processor cause the at least one processor to: determine, for each topic having a combined relevance, whether the user's interest in the topic is significant by comparing the topic's combined relevance with the user-interest threshold; and for each topic determined to be of significant interest to the user, use the topic to perform a per interest retrieval, the per interest retrieval retrieving content items in the plurality of content items that have a relationship with the topic; group any remaining topics, and perform a group interest retrieval to retrieve content items in the plurality of content items, each content item retrieved via the group interest retrieval having a relationship with all of the grouped topics.