Card sorting is a simple technique in user experience design where a group of subject experts or "users", however inexperienced with design, are guided to generate a category tree or folksonomy. It is a useful approach for designing information architecture, workflows, menu structure, or web site navigation paths.
Card sorting has a characteristically low-tech approach. The concepts are first identified and written onto simple index cards or Post-it notes. The user group then arranges these to represent the groups or structures they are familiar with.
A card sort is commonly undertaken when designing a navigation structure for an environment that offers an interesting variety of content and functionality, such as a web site. In that context, the items to be organized are those that are significant in the environment. The way that the items are organized should make sense to the target audience and cannot be determined from first principles.
The field of information architecture is founded upon the study of the structure of information. If an accepted and standardized taxonomy exists for a subject, it would be natural to simply apply that taxonomy as a means of organizing both the information in the environment and any navigation to particular subjects or functions. Card sorting is applied when:
- The variety in the items to be organized is so great that no existing taxonomy is accepted as organizing the items.
- The similarities among the items make them difficult to divide clearly into categories.
- Members of the audience that uses the environment may differ significantly in how they view the similarities among items and the appropriate groupings of items.
Basic method 
To perform a card sort:
- A person representative of the audience is given a set of index cards with terms already written on them.
- This person puts the terms into logical groupings, and finds a category name for each grouping.
- This process is repeated across a population of test subjects.
- The results are later analyzed to reveal patterns.
Open card sorting 
In an open card sort, participants create their own names for the categories.
This helps reveal not only how they mentally classify the cards, but also what terms they use for the categories.
Open sorting is generative; it is typically used to discover patterns in how participants classify, which in turn helps generate ideas for organizing information.
Closed card sorting 
In a closed card sort, participants are provided with a predetermined set of category names. They then assign the index cards to these fixed categories.
This helps reveal the degree to which the participants agree on which cards belong under each category.
Closed sorting is evaluative; it is typically used to judge whether a given set of category names provides an effective way to organize a given collection of content.
Reverse card sorting 
See tree testing.
In a reverse card sort or card-based classification test, an existing structure of categories and sub-categories is tested. Users are given tasks and are asked to complete them navigating a collection of cards. Each card contains the names of subcategories related to a category, and the user should find the card most relevant to the given task starting from the main card with the top-level categories. This ensures that the structure is evaluated in isolation, nullifying the effects of navigational aids, visual design, and other factors.
Reverse card sorting is evaluative; it's used to judge whether a predetermined hierarchy provides a good way to find information.
Analyzing card-sort results 
Various methods can be used to analyze the data. The purpose of the analysis is to extract patterns from the population of test subjects, so that a common set of categories and relationships emerges. This common set is then incorporated into the design of the environment, either for navigation or for other purposes. Card sorting is also evaluated through dendrograms. There is some indication that different evaluation method for card sorting provide different result.
Card sorting is an established technique with an emerging literature.
Online (remote) card sorting 
There are a number of tools available to perform card sorting activities with survey participants via the internet. The perceived advantage of remote card sorting is that it allows a larger group of participants to be reached at a lower cost. The software can also assist in the process of analyzing card sort results. The advantages of a remote card sort must be traded off against the lack of personal interaction between card sort participants and the card sort administrator, which may produce valuable insights.
- Jakob Nielsen (May 1995). "Card Sorting to Discover the Users' Model of the Information Space".
- Jakob Nielsen (July 19, 2004). "Card Sorting: How Many Users to Test".
- Donna Maurer and Todd Warfel. "Card sorting: a definitive guide".
- "Design for Usability - Card Sorting". Syntagm Ltd.
- Head First Web Design. O'Reilly Media. 2009. pp. 81–100. ISBN 978-0-596-52030-4.
- Nawaz, Ather (09 2012). "A Comparison of Card-sorting Analysis Methods". The 10th Asia Pacific Conference on Computer Human Interaction (APCHI2012).
- Spencer (2009). "Card Sorting (". Rosenfeld Media. Unknown parameter
- Peer-reviewed encyclopedia chapter on Card Sorting
- Short video explanation of card sorting
- OptimalSort—web-based tool with results analysis.
- Simple Card Sort—online card-sorting tool with results analysis.
- SynCaps—computer-aided paper sorting tools and videos
- UsabiliTEST—web-based tool with built-in data analytics.
- WebSort—web-based tool with results analysis.
- ConceptCodify—web-based tool with results analysis.