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How Many Users Does it
Take to Change a Web Site?

(SIGCHI Bulletin May/June 2001)

Well, naturally this is a trick question, but it has recently become a little trickier. Ever since Nielsen and Landauer's InterCHI 1993 paper, the answer has been "about five". This is based on a mathematical model that projects the number of new problems that each additional usability test subject will find. Five users should find about 85% of usability problems with significantly diminishing returns thereafter.

But by the time you read this, Jared Spool and his colleagues will have presented a short paper at CHI 2001 entitled Five Users is Nowhere Near Enough, reporting on a study that failed to find even half of a web site's predicted usability problems with a whopping 18 users. So where does that leave us with our little conundrum?

Let's consider why this kind of discrepancy might exist. Traditionally, usability testing has been performed using well-defined tasks. Spool's team, on the other hand, conducted what might best be described as "goal-directed" tests. In these studies rather than giving users very specific scenarios, they were asked simply to buy something they needed (CD's and videos for example). This meant that users were formulating their own sub-goals and tasks. It also meant that the detail with which they described their goals could have varied from something vague like "a CD my mom will like" to a specific requirement for "the Erin Brockovich video" (hopefully spelled correctly).

Given these potential variations, it is likely that users would have been testing different parts of the same web site. Some users may have required the search facility, others might have tried browsing to find that CD for mom. The issue here, to use the conventional software testing term, is coverage. The Nielsen and Landauer model only works because users are exposed to the same aspects of the system under test. Regrettably, given the complexity of some web pages, we cannot even be sure which aspects of the system users are being exposed to. Parts of a page that one user may dismiss as irrelevant might be examined by another in minute detail as they see a vital clue in the pursuit of their goal.

All of this has some worrying implications for web site usability testing. On the one hand, insisting that users focus on a well-defined, detailed task usually only requires about five subjects per test. On the other, is this a realistic way to determine the overall usability of site? As if to pre-empt this question, Jakob Nielsen has recently published an Alertbox called Success Rate: The Simplest Usability Metric. In it he makes the point that the best methods for usability testing (with small numbers of users) conflict with the demands of getting an overall measure of a site's usability. The latter can only be done (with confidence) by measuring the success rates of larger numbers of users in achieving their goals.

The consequence of this fits the five-versus-eighteen-plus puzzle together rather nicely. We can use small numbers of subjects in detailed and well-focused tests, ensuring that coverage is the same within a series. On large or complex sites we will need more test series to improve coverage. However, to get a global picture of a site's usability, we need to measure the success rate of real users in real situations.


The mathematical model used by Nielsen and Landauer was devised by Bob Virzi at GTE. (Thanks to Wayne Gray for pointing out this oversight in the original article.)

R.A. Virzi (1992). 'Refining the test phase of usability evaluation: How many subjects is enough?' Human Factors, 34(4), 457-468.

The Author

William Hudson is principal consultant for Syntagm Ltd, based near Oxford in the UK. His experience ranges from firmware to desktop applications, but he started by writing interactive software in the early 1970's. For the past ten years his focus has been user interface design, object-oriented design and HCI.

Other free articles on user-centred design: www.syntagm.co.uk/design/articles.htm

© 2001-2005 ACM. This is the author's version of the work. It is posted here by permission of ACM for your personal use. Not for redistribution. The definitive version was published in SIGCHI Bulletin, {Volume 33, May-June 2001} http://doi.acm.org/10.1145/967222.967230

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