Textbook of Usability

Card sorting is a user research method in which participants group items (written on cards) into categories that make sense to them. The results reveal users' mental models of content organisation, guiding information architecture decisions.

Two main variants:

• Open card sort: participants create their own categories and name them, revealing unfiltered mental models
• Closed card sort: participants sort into predefined categories, testing whether a proposed structure matches user expectations
• Hybrid card sort: predefined categories are offered but participants can add their own

Card sorting can be conducted in person (physical index cards on a table) or remotely using tools like Optimal Workshop. Typical sample sizes are 15–30 participants, enough to reveal consistent groupings while accommodating individual variation.

Analysis identifies:

• Consistently grouped items (candidates for the same category)
• Contested items (belonging to different categories for different users — may need to appear in multiple places or require cross-linking)
• Category names (which labels users actually use)
• Dendrograms showing the hierarchical relationships users perceive

Card sorting is usually paired with tree testing, which validates a proposed hierarchy by asking users to find specific items within it. Together they ensure information architecture matches user mental models before interface development begins.