There was a good thread on the IAI Members List recently about expanding results during faceted search. For those not on the list, here are a few highlights:
Q. I'm restructuring the faceted search sidebar on an e-commerce search results page. What's your opinion about the user being able to expand results within a given search term, as well as narrow them?
A1. I do think it's worth enabling users to expand as well as narrow. NCSU's stacking breadcrumbs are an excellent way to show context and support relaxing one or more criteria. When I'm using Amazon, I often begin with a query (e.g., hitch rack), then navigate the category structure (Sports & Outdoors > Car Sports Racks > Bike Racks), and then wish I could remove my original query term (so I can see all items in the category, not just hitch racks), but with Amazon, I can't do that easily.
A2. In some usability studies we've done (at NCSU Libraries) we found that without prior training, a small percentage of users take advantage of this de-selection functionality. Most folks seem to ignore it. For those that "get it" it can be quite a handy tool for managing a large result set as it allows the user to expand and contract with minimal interaction...The larger the catalog, the more I think facet management tools (like de-selection buttons) add value.