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Card Sorting: How Many Users to Test

by Jakob Nielsenon July 19,2004

Summary:Testing ever-more users in card sorting has diminishing returns,but test at least 15 users — 3 times more than you would in traditional usability tests.

One of the biggest challenges in website and intranet manbetx官方网站手机版design is creating theinformation architecture: what goes where?A classic mistake is to structure the information space based on howview the content— which often results in different subsites for each of your company's departments or information providers.

Rather than simply mirroring your org chart,you can better enhance usability by creating an information architecture that reflects howusersview the content.In each of ourintranet studies,we've found that some of the biggestproductivity gains occur whencompanies restructure their intranetto reflect employees' workflow.And inecommerce,sales increase when products appear in the categories where users expect to find them.

All very good,buthow do you find outthe users' view of an information space and where they think each item should go?Forresearching this type of mental model,the primary method is card sorting:

  1. Write the name (and perhaps a short description) of each of the main items on an index card.Yes,good old paper cards.(Taking care not to useterms that biasthe users.)
  2. Shuffle the cards and give the deck to a user.(The standardrecommendations for recruiting test participantsapply: they must be representative users,etc.)
  3. Ask each user to sort the cards into piles,placing items that belong together in the same pile.Users can make as many or as few piles as they want;some piles can be big,others small.
  4. Optional extra steps include asking users to arrange the resulting piles into bigger groups,and to name the different groups and piles.The latter step can give you ideas for words and synonyms to use for navigation labels,links,headlines,and search engine optimization.

Because card sorting uses no technology,thisphoto of a 1995 card sortlooks the same as one conducted today.

Research Study

Fidelity Investments has one of the world's best usability teams,led by Dr.Thomas S.Tullis,senior VP of human interface manbetx官方网站手机版design.Tullis and co-author Larry Wood recently reported the results of a study measuring the trade-off curve for testing various numbers of users in a card sorting exercise.

First,they tested 168 users,generating very solid results.They then simulated the outcome of running card sorting studies with smaller user groups by analyzing random subsets of the total dataset.For example,to see what a test of 20 users would generate,they selected 20 users randomly from the total set of 168 and analyzed only that subgroup's card sorting data.By selecting many such samples,it was possible to estimate the average findings from testing different numbers of users.

The main quantitative data from a card sorting study is a set ofsimilarity scoresthat measures the similarity of user ratings for various item pairs.If all users sorted two cards into the same pile,then the two items represented by the cards would have 100% similarity.If half the users placed two cards together and half placed them in separate piles,those two items would have a 50% similarity score.

We can assess the outcome of a smaller card sorting study by asking how well its similarity scores correlate with the scores derived from testing a large user group.(A reminder: correlations run from -1 to +1.A correlation of 1 shows that the two datasets are perfectly aligned;0 indicates no relationship;and negative correlations indicate datasets that are opposites of each other.)

How Many Users?

For most usability studies,I recommendtesting 5 users,since that's enough data to teach you most of what you'll ever learn in a test.For card sorting,however,there's only a 0.75 correlation between the results from 5 users and the ultimate results.That's not good enough.

You must test 15 users to reach a correlation of 0.90,which is a more comfortable place to stop.After 15 users,diminishing returns set in and correlations increase very little: testing 30 people gives a correlation of 0.95 — certainly better,but usually not worth twice the money.There are hardly any improvements from going beyond 30 users: you have to test 60 people to reach 0.98,and doing so is definitely wasteful.

Tullis and Wood recommend testing 20–30 users for card sorting.Based on their data,my recommendation is to test 15 users.

Why do I recommend testing fewer users?I think that correlations of 0.90 (for 15 users) or maybe 0.93 (for 20) are good enough for most practical purposes.I can certainly see testing 30 people and reaching 0.95 if you have a big,well-funded project with a lot of money at stake (say,an intranet for 100,000 employees or an ecommerce site with half a billion dollars in revenues).But most projects have very limited resources for user research;the remaining 15 users are better "spent" on 3 qualitative usability tests of different manbetx官方网站手机版design iterations.

Let Users Inform Your manbetx官方网站手机版Design

I don't recommend manbetx官方网站手机版designing an information architecture based purely on a card sort's numeric similarity scores.When deciding specifics of what goes where,you should rely just as much on the qualitative insights you gain in the testing sessions.Much of the value from card sorting comes fromlistening to the users' commentsas they sort the cards: knowingwhypeople place certain cards together gives deeper insight into their mental models than the pure fact that they sorted cards into the same pile.

Why More Users for Card Sorting?

We know that 5 users are enough for most usability studies,so why do we need three times as many participants to reach the same level of insight with card sorting?Because the methods differ in two key ways:

  • User testing is anevaluation method: we already have a manbetx官方网站手机版design,and we're trying to find out whether or not it's a good match with human nature and user needs.Although people differ substantially in their capabilities (domain knowledge,intelligence,and computer skills),if a certain manbetx官方网站手机版design element causes difficulties,we'll see so after testing a few users.A low-end user might experience more severe difficulties than a high-end user,but the magnitude of the difficulties is not at issue unless you are running a measurement study (which requires more users).All you need to know is that the manbetx官方网站手机版design element doesn't workfor humans and should be changed.
  • Card sorting is agenerative method: we don't yet have a manbetx官方网站手机版design,and our goal is to find out how people think about certain issues.There is great variability in different people's mental models and in the vocabulary they use to describe the same concepts.We must collect data from a fair number of users before we can achieve a stable picture of theusers' preferred structure and determine how to accommodate differencesamong users.

如果你有一个现有的网站内部,testing a few users will tell you whether people have trouble with the information architecture.To generate a new structure from scratch,you must sample more people.

Luckily,you cancombine the two methods: First,use generative studies to set the direction for your manbetx官方网站手机版design.Second,draft up a manbetx官方网站手机版design,preferably usingpaper prototyping,and run evaluation studies to refine the manbetx官方网站手机版design.Because usability evaluations are fast and cheap,you can afford multiple rounds;they also provide quality assurance for your initial generative findings.This is why you shouldn't waste resources squeezing the last 0.02 points of correlation out of your card sorts.You'll catch any small mistakes in subsequent user testing,which will be much cheaper than doubling or tripling the size of your card sorting studies.

Study Weaknesses

The Fidelity study has two obvious weaknesses:

  • It's only one study.It's always better to have data from multiple companies.
  • The analysis was purely quantitative,focusing on a statistical analysis of similarity scores and ignoring user comments and other qualitative data.

These two weaknesses are not fatal.I view this as a pioneering study and a great contribution to our web usability knowledge.But,because of the study's weaknesses,it would be useful if somebody duplicated it with different information spaces,and also analyzed the qualitative data along with the numeric scores.Sounds like a good thesis project for a graduate student who's looking to research something with real-world impact (hint,hint).

Even though more data would be comforting,I have confidence in the Fidelity study's conclusions because they match my own observations from numerous card studies over many years.I've always said that it was necessary to test more users for card sorting than for traditional usability studies.And I've usually recommended about 15 users,though we've also had good results with as few as 12 when budgets were tight or users were particularly hard to recruit.

There are a myriad ways in whichquantitative studies can go wrongand mislead you.Thus,if you see a single quantitative study that contradicts all that's known from qualitative studies,it's prudent to disregard the new study and assume that it's likely to be bogus.But when a quantitative study confirms what's already known,it's likely to be correct,and you can use the new numbers as decent estimates,even if they're based on less data than you would ideally like.

Thus,the current recommendation is totest 15 users for card sortingin most projects and 30 users in big projects with lavish funding.


Tullis,Tom,and Wood,Larry.(2004)How Many Users Are Enough for a Card-Sorting Study?,Usability Professionals Association (UPA) 2004 Conference,Minneapolis,MN,June 7–11,2004.