01 / THE IDEA
What does it mean?
To measure how common a particular error is, define what counts as that error before the test. Then divide the number of participants who made it by the number who attempted the task. Track different errors separately when they have different consequences.
02 / IN PRACTICE
Make it concrete.
Example: 10 participants attempt checkout and 3 select the wrong delivery address. In this test, 30% made that specific error.
03 / WHAT TO WATCH FOR
A common mistake.
Counting all wrong clicks equally, regardless of their impact on the user.
04 / WHEN IT HELPS
When should you use it?
When mistakes introduce risk, rework, or loss of trust.
05 / FURTHER READING
Explore the source.
Nielsen Norman Group — Success Rate: The Simplest Usability Metric ↗This is a reference for further learning, not an endorsement or a verbatim quotation.