Why You Only Need to Test with 5 Users

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Some people think that usability is very costly and complex and that user tests should be reserved for the rare web design project with a huge budget and a lavish time schedule. Not true. Elaborate usability tests are a waste of resources. The best results come from testing no more than 5 users and running as many small tests as you can afford.

In earlier research, Tom Landauer and I showed that the number of usability problems found in a usability test with n users is:

N (1-(1- L ) n )

where N is the total number of usability problems in the design and L is the proportion of usability problems discovered while testing a single user. The typical value of L is 31%, averaged across a large number of projects we studied. Plotting the curve for L =31% gives the following result…

Diminishing returns for usability testing, as more and more users are tested. The curve bends around 5 users, which is the recommended number of test participants.

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