Task Success and Time on Task, Done Properly
Two core usability metrics: whether participants complete a task correctly, and how long they take to do it.
Priya Raghunathan
The key figures
- Task success
- the percentage of participants who complete a task
- Partial success
- can be scored separately where tasks have degrees of completion
- Time on task
- often reported only for successful attempts
- Sample size
- reliable metrics need larger studies than qualitative testing
Why this is worth getting right
They turn usability into numbers that can be compared across versions and connected to business outcomes.
Do this, not that
Do
- Define success criteria before testing
- Record failures and partial successes separately
- Measure time only for comparable tasks
- Compare against a baseline
- Report confidence intervals for quantitative studies
Don’t
- Changing success criteria after the test
- Comparing times across different tasks
- Drawing conclusions from very small samples
- Ignoring why failures happened
When to bring in help
Our advice Bring in help when setting up UX benchmarking, or when metrics must support a business case.
Where this comes from
- Nielsen Norman Group — Success rate: the simplest usability metric
- Nielsen Norman Group — Quantitative studies: how many users to test
The figures and practices above come from the sources listed.
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Where to go next
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