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What Is a Confidence Interval and Why Would You Want One? :: UXmatters

What is a confidence interval? I wanted to know that recently and turned to one of my favorite books: Measuring the User Experience, by Tom Tullis and Bill Albert. And here’s what they say:

“Confidence intervals are extremely valuable for any usability professional. A confidence interval is a range that estimates the true population value for a statistic.”

Then they go on to explain how you calculate a confidence interval in Excel. Which is fine, but I have to admit that I wasn’t entirely sure that once I’d calculated it, I really knew what I’d done or what it meant. So I trawled through various statistics books to gain a better understanding of confidence intervals, and this column is the result.

The Starting Point: The Need for a Measurement

Are you more comfortable working with qualitative data than quantitative data? If so, you’re like most UX people—including me. Once we’ve seen three or four test participants in a row fail for the same reason, we just want to get on with fixing the problem.

But sooner or later, we’ll have to tangle with some quantitative data. Let’s say, for example, that we have this goal for a new product: On average, we want users to be able to do a key task within 60 seconds. We’ve fixed all the show-stoppers and tested with eight participants—all of whom can do the task. Yay! But have we met the goal? Assuming we remembered to record the time it took each participant to complete the task, we might have data that looks like this:

Task times for 8 individuals

Continue reading this article at: What Is a Confidence Interval and Why Would You Want One? :: UXmatters.