User testing numbers

Jakob Nielsen suggests only testing 3-5 users because of the decreasing return on investment for more users.

Increase in proportion of usability problems found as a function of number of users tested

The graph from his post suggests that you find most of the errors in user interfaces with in the first few users. He suggests testing five users, then redesigning based on the feedback, then testing five more. It increases quality of data and results in a better product while keeping budget costs under control. It's a more efficient way to go.

The exception is when you have distinct user groups, in which you test 3-4 users of each distinct user group in the same fashion as noted above.

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