Fortunately, there is a way to take this into account. Binary success rates are essentially proportions: the proportion of users who completed a given task successfully. For example, if 5 of the 10 participants completed a task, the success rate is 5/10 = 0.5. The appropriate way to calculate a confidence interval for a proportion like this is to use a binomial confidence interval. Several methods are available for calculating binomial confidence intervals, such as the Wald Method and the Exact Method. But as Sauro and Lewis (2005) have shown, many of those methods are too conservative or too liberal in their calculation of the confidence interval when dealing with the small samp...
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