Designing for Faceted Search

by Stephanie Lemieux

This article was originally published in KM World, March 1, 2009.

Faceted Search-Keeping Up With the Jones’

Faceted search, or guided navigation, has become the de facto standard for e-commerce and product-related websites, from big box stores to product review sites. But e-commerce sites aren’t the only ones joining the facets club. Other content-heavy sites such as media publishers (e.g. Financial Times), libraries (e.g. NCSU Libraries), and even non-profits (e.g. Urban Land Institute) are tapping into faceted search to make their often broad-range of content more findable. Essentially, faceted search has become so ubiquitous that users are not only getting used to it, they are coming to expect it.

Faceted Search Explained… Briefly

Faceted search lets users refine or navigate a collection of information by using a number of discrete attributes-the so-called facets. A facet represents a specific perspective on content that is typically clearly bounded and mutually exclusive. The values within a facet can be a flat list that allows only one choice (e.g. a list of possible shoe sizes) or a hierarchical list that allow you to drill-down through multiple levels (e.g. product types, Computers > Laptops). The combination of all facets and values are often called a faceted taxonomy. These faceted values can be added directly to content as metadata or extracted automatically using text mining software.

The power of faceted search lies in the ability of users to create their own custom navigation by combining various perspectives rather than forcing them through a specific path. Think of a cookbook: authors have to organize the recipes in one way only-by course or by main ingredient-and users have to work with whatever choice of organizing principle that has been made, regardless of how that fits their particular style of searching. An online recipe site using faceted search can allow users to decide how they’d like to navigate to a specific recipe, offering multiple entry points and successive refinements. The diagram shows how facets can help pinpoint content very precisely through the combination of perspectives. Just 3 facets with 5 terms each can represent 243 possible combinations.

Fig. 1: Combinatorial facets for a recipe site

As users combine facet values, the search engine is really launching a new search based on the selected values, which allows the users to see how many documents are left in the set corresponding to each remaining facet choice. So while users think they are navigating a site, they are really doing the dreaded advanced search-without the scary librarians-only interface.

Design Dos & Don’ts in Faceted Search

Now that faceted search interfaces are so prevalent, patterns are emerging that establish good design. If you are considering embarking on a faceted search implementation, here are 5 important points to consider:

DON’T go crazy with facets. Information overload is bad enough in general–don’t add to it by presenting users with 15 different facets. This is hardly “narrowing”, and users will generally not scroll too far down beyond the initial screen to locate your more obscure facets.

But how do you make sure your facets are focused and helpful?

DO base facets on key use cases & known user access patterns. A little bit of research goes a long way in identifying key ways users navigate and search your site. Analysing search logs, evaluating competitor sites, and user research and testing are great ways to figure out what key access points users are looking for. Interviewing as few as 10 users will often give you great insight into what the facet structure should be. Don’t skimp on this up front research, you’ll thank yourself later as you continuously refer back to this data while you configure your taxonomy and search engine.
DO order facets & values based on importance. This might sound obvious, but a lot of sites get this wrong. Not all facets are created equal: some access points are more important than others depending on what users are doing and where they are in the site. Give them top billing as only the first few will be visible on page load. Same goes for values: most faceted search engines will allow you to order values based on number of items in that category. This is almost always a better bet than alphabetical ordering, as it dynamically presents the most popular items at the top. When determining order for navigation, again think about your users and why they are coming to your site: don’t obscure the big-play items in an alpha scheme.
DO leverage the tool to show & hide facets and values. While the free or low-cost faceted search tools don’t all offer these configuration options, more sophisticated faceted search solutions allow you to create rules to progressively disclose facets. Think of a site offering online greeting cards. While the visual theme of the card-teddy bears, a sunset, golf-might eventually be important to a user, it probably isn’t the first place they will start their search. They will likely start with occasion (birthday, Christmas), or recipient (father, friend), and then become interested in themes further down the line. Accordingly, we might hide the “themes” facet until a user has selected an occasion or recipient. You can selectively present facets based on your understanding of your users and their typical search patterns (as mentioned in the previous “do”).

Also take advantage of the search engine’s clutter-reducing features, such as the “more…” link. This allows you to present only the most popular items and hide the rest until the user specifically requests to see them. You can also do this at the facet level, collapsing lesser-used facets to present just the category name and let users who are interested expand that facet.

Richard Tomlinson, solution architect at Endeca, advises that facet display should be dependent on the area of the site. If you are in the first few layers of your site, you should show fewer facets with more values exposed, whereas if you are deeper into product information you should show more facets, some with values exposed and others hidden.

DO build your taxonomy with faceted search in mind. This is a big one, because a good taxonomy goes a long way in making a successful faceted search interface. There are some important guidelines to follow in taxonomy design. Facets need to be well defined, mutually exclusive and have clear labels. For example, having one facet called “Training” and another “Events” is confusing: where do you put a seminar? Is it training or an event? If you have to wonder, your users will too. The taxonomy depth (how many levels deep does it go) and breadth (how many facets wide is it) are other important considerations. Faceted search works better with a broad taxonomy that is relatively shallow, as this lets users combine more perspectives rather than get stuck in an eternal drill down, which causes fatigue. The facet configuration and display rules will help you create the optimal progressive presentation of these facets so as to not overwhelm users with the breadth.

Broad vs deep taxonomy — Fig 2: Broad vs. deep taxonomy

If you are torn between two places in the taxonomy for a term, consider putting it in both places. This is called polyhierarchy, and it is a good way to ensure findability from multiple perspectives (although faceted search vendors aren’t too keen on the concept as their tools sometimes don’t handle polyhierarchy terribly well). One caveat: polyhierarchy is best served within a facet rather than across multiple facets. Since facets should be mutually exclusive, you shouldn’t have much need to repeat terms across facets, which can be more confusing than helpful.

The most important thing however, is to be prepared to break any of these rules in the name of usability. Essentially, building a faceted taxonomy is more of an art than a science and it involves understanding your users’ search behaviors.

Faceted Search Trends

As the trend towards increased social computing continues, Web 2.0 concepts are entering the realm of faceted search. We are starting to see social tags being used in faceted search and browse interfaces. Buzzillions.com, a product-review site, is using social tag-based facets in its navigation, allowing users to refine results based on tags grouped as “Pros” or “Cons”.

This site uses a nice blend of free social tagging and control to ensure good user experience; when you type in a tag to add to a product review, type-ahead verifies existing tags and prompts you to select one from the existing list of matches to maximize consistency.

Doing it Right

Ultimately, navigation and search is one of the main interactions users have with your site, so getting it right is not just a matter of good design, it impacts the bottom line. Faceted search is a very popular and powerful solution when done well; it allows users to deconstruct a large set of results into bite-size pieces and navigate based on what’s important to them. But faceted search by itself is not necessarily going to make your users lives easier. You need to understand your users’ mental models (how they seek information), test your assumptions about how they will interpret your terms and categories and spend time refining your approach.

Faceted search can just add more complexity and frustrate your users if not considered from the user perspective and carefully thought through with sound usability principles in mind. Faceted search is raising the bar in terms of findability and how well you execute will determine whether your site meets the new standard.

Published here on April 28, 2009.