The card sorting method is used to generate information about the associations and grouping of specific data items. Participants in a card sort are asked to organize individual, unsorted items into groups and may, depending on the technique, also provide labels for these groups.
In a user-centered design process, it is commonly used when developing a site architecture but has also been applied to developing workflows, menus, toolbars, and other elements of system design.
Sorting and grouping have long been studied within psychology and the research dates back at least to the 1950s. Numerous, non-peer reviewed descriptions, case studies, and blogs have been written in the last several years on the technique and its use in the user-centered design process, but only a few peer reviewed articles on the technique have been published and little is known of its validity or reliability as a means of directly producing a useful and usable architecture. Instead, card sorts are generally used to provide insight that is used by a practitioner to generate an architecture.
The Card sorting technique originates in cognitive psychology techniques. One of most significant of the methods is known as the Wisconsin Card Sorting Task (WCST), which was introduced in 1946. The WCST is quick and easy to administer, requiring no specialized equipment but the deck of cards itself and a book to compare scores. It's used to determine competence with abstract reasoning, and ability to change problem solving strategies when needed.
Benefits, Advantages and Disadvantages
Card sorting can be used to:
There are three common types of card sorting for examining how users organize information: Open, closed, and inverse:
Open Card Sort. The purpose of this card sort is to explore organizational models for new information or information which is difficult to classify. In an open card sort, participants are provided with a set of items to sort and asked to generate groups, arrange groups into a hierarchical structure, and provide the labels for the groups. Some open card sorts also allow the participants to suggest additional content or to eliminate content from the sorted results. This type of card sort is generally used in the earliest phases of a design project.
Closed Card Sort. The purpose of this card sort is to explore how new content may fit into an existing structure, or how new content relates to existing content. In a closed card sort, participants are provided a set of items to sort and pre defined categories and are asked to place the items into the predefined categories. The primary focus of this type of card sort is to evaluate the contents of the groups and their labels. This type of card sort is generally used in the later phases of a design project after an open card sort or to restructure existing architectures.
Inverse Card Sort. Open and closed card sorting are a “bottom up” processes since participants determine a logical structure or evaluate a structure after viewing all of the items that will make up the structure. However, the activity of discovering the location of items within a structure is a top down process where the entire content set of the structure is often unknown to the user. For this reason, an Inverse (also known as a Reverse Card Sort) activity may be used to evaluate a proposed structure. In an Inverse Card Sort activity, participants are asked to try to locate a specific item within a completed structure. This is, essentially, a usability evaluation of a completed card sort but focuses exclusively on the structure and labeling. The primary goal of this type of card sort activity is to evaluate a structure in a format more closely aligned with its actual use.
Number of Participants
Most card sort activities are of a much smaller scale and often include some discussion between the participant and a facilitator, between multiple participants, or both. In fact, some practitioners recommend only doing card sorting as a group activity even if individual card sorting is included. Kaufman  recommends “at least ten participants” for a card sort exercise but cites no data for this recommendation and even states that “you can achieve reasonable results with fewer”. Tullis and Wood  concluded that ” reasonable structures are obtained from 20-30 participants” based on analysis of data from a project involving 168 participants in a card sort study. Neilson  recommends performing cards sorts with 15 based on an analysis of the same data used by Tullis and Wood. Paul  suggests that a reasonable structure can be generated using as few as 5 participants if the card sort exercise is part of a Delphi-based research approach. However, none of the works cited discuss the effect of multiple potential user groups providing input, how to address potentially different but equally viable architectures, or how the natural similarities or differences in the data affect the number of participants needed. As a result, no clear evidence exists on an appropriate number of participants to use. Since the most common use of this technique is to generate data to help designers understand the user’s perspective on organization but not to actually generate a proposed architecture, expert assessment is needed to determine when sufficient data has been collected.
Informants should be representative of the user population for whom the application is being designed.
For a physical card sort:
For an online card sort:
A real advantage of online card sorting software is the ability for the software to tally and post results for each participant. A disadvantage of online card sorting software is that many of the tools don't allow participants to create subcategories.
Who Can Facilitate
For on-site card sorting, an experienced facilitator with knowledge of card sorting procedures is recommended. The facilitator is responsible for briefing the participant, explaining the procedures, and answering questions without affecting the participants' responses.
Data Analysis Approach
Costs and Scalability
Most card sorts are relatively inexpensive and online card sorting software allow large samples of data to be collected efficiently. The major issue with scalability involves the number of cards that participants must sort in what is usually one to two hours (unless the participants are very dedicated). Recommendations on how many cards people can sort range from about 20 to 200. If there are more than 200 cards, then some type of sampling might be effective (Nakhimovsky, Schusteritsch, & Rodden, 2006). Some of the online card sorting tools become unwieldy around 100 items so that might affect scalability.
Participants who are familiar with the items on the cards can probably sort a larger sample than people less familiar with the content.