Glossary
Task success rate
What is task success rate?
Task success rate, also called task completion rate, is the proportion of eligible task attempts that meet a defined success criterion in a usability study.
How to measure it
Define success before testing. A completed task, a task completed with help, and a partially completed task are different outcomes. State whether the measure is binary or graded.
For a binary measure, divide successful attempts by eligible attempts and multiply by 100. If 8 of 10 attempts meet the criterion, the observed success rate is 80%.
Sauro's analysis reported a 78% reference value across 1,189 tasks. That is a comparison point from a dataset, not the required rate for every task. Report the sample and uncertainty, and consider time on task and errors alongside completion.
Example
A portal study might find lower completion for filing a claim than for paying a bill. That deserves investigation. The priority also depends on how important the task is, how certain the comparison is, and what failure costs the user.
Why it matters
Can users actually do what they came to do? Task success rate helps answer that question for the task and conditions tested.