Enter successes and attempts per task to compute the completion rate with a 95% confidence interval (Adjusted-Wald). Supports binary scoring or partial-credit for tasks with subcomponents.
Task success rate is the share of participants who completed a task — one of the three canonical ISO 9241 usability metrics. For moderated tests, use binary scoring unless the task has clear subcomponents. For benchmark studies, partial-credit scoring reduces floor and ceiling effects (Sauro).
The 95% CI widens dramatically for small samples. 10 participants, 8 successes (80%) gives a CI of roughly 50–95%. 100 participants, 80 successes narrows to 71–87%. Always report the CI alongside the point estimate.
The Adjusted-Wald (Agresti-Coull) interval handles small samples better than the standard normal approximation. For 0% or 100% cases, it still produces reasonable bounds.
Related: SUS calculator, sample size calculator, usability testing guide.