How 3 users can tell you if something is unusable

in Usability testing

Three users can be enough to tell you whether something is unusable.

Imagine this: you ask three users to find something specific on a website. All three of them fail. The task success rate is 0 out of 3.

Given this, the average task success rate will almost certainly be less than 65% if all your target users were tested. That is, with just 3 users, we can statistically forecast that most users would not manage to do a third of the tasks asked of them. That’s not good – you should be aiming for at least 80%.

How can you use this knowledge? Well, if someone is telling you that something is fit to go live, do a test with three target users. Get them to do one core task each. If they all fail the task, you can be almost certain (95% certain to be exact) that it shouldn’t go live just yet and that more usability work is needed.

Caution: this three-user test tells you whether something is not fit for purpose.

What if the users manage to do all three tasks? It tells you that the average success rate will be somewhere between 38% and 100%. That’s the most you can say with a sample of three users. You’d need to test with more users to be more specific about how good the product is. Find out more about quantitative studies.

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