What is Task success rate?
Task success rate is the percentage of users who successfully complete a given task in a usability study. It is the most fundamental usability metric, directly measuring whether a design enables users to accomplish their goals.
How it works
Task success can be measured as binary (completed/not completed) or graded (completed without help, completed with hints, partially completed, failed). Sauro and Lewis recommend 78% as a benchmark for binary success rate based on meta-analysis of hundreds of usability studies. Task success rate is most meaningful when combined with time on task (efficiency) and error rate (quality of the path). A high success rate with high task time suggests the design works but is inefficient. A low success rate identifies tasks that need redesign. Sample sizes of 5-8 users per task are typical for qualitative testing; larger samples are needed for statistical comparisons.
Applied example
A usability study of a new self-service portal shows that ‘Pay a bill’ has a 95% success rate, ‘Update contact information’ has 85%, but ‘File a claim’ has only 40%. The 40% rate flags the claims process as the priority redesign target, focusing resources on the area with the greatest usability gap.
Why it matters
Task success rate is the most direct measure of whether a design serves its purpose, cutting through subjective opinions to answer the fundamental question: can users actually do what they came to do?
