7. The system of claim 6 , where, when receiving the one or more selections of the category listings, the one or more computer devices are configured to: receive an instruction to add a first one of the category listings, corresponding to first subject matter that is on-topic relative to a topic of the search query, to a positive preferences list, and receive an instruction to add a second one of the category listings, corresponding to second subject matter that is off-topic relative to the topic of the search query, to a negative preferences list.
7. The system of claim 6 , where, when receiving the one or more selections of the category listings, the one or more computer devices are configured to: receive an instruction to add a first one of the category listings, corresponding to first subject matter that is on-topic relative to a topic of the search query, to a positive preferences list, and receive an instruction to add a second one of the category listings, corresponding to second subject matter that is off-topic relative to the topic of the search query, to a negative preferences list. 8. The system of claim 7 , where, when including the one of the references in the filtered set of references, the one or more computer devices are configured further to: include, in the filtered set of references, one of the references, corresponding to the first one of the category listings, from the positive preferences list, and exclude, from the filtered set of references, one of the references, corresponding to the second one of the category listings, from the negative preferences list.