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Card Sorting: How Many Users to Test (19 Jul 2004)
There are myriad ways in which quantitative studies can go wrong and 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 to test fifteen users for card sorting in most projects, and thirty users in big projects with lavish funding.
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