Textbook of Usability

A severity rating is a systematic prioritisation of identified usability problems to guide remediation. Rather than treating all problems equally, severity ratings combine measures of impact and frequency to identify which problems should be fixed first.

Jakob Nielsen's widely used severity scale:

1. Cosmetic — noticed by some users but does not affect task performance
2. Minor — causes minor delay or confusion but users recover
3. Major — causes significant difficulty; some users fail the task
4. Critical — prevents task completion for most users; may cause data loss or safety risk

A more nuanced framework combines two dimensions explicitly:

• Impact: how serious is the problem when it occurs? (cosmetic, minor, major, critical)
• Frequency: how often will users encounter it? (rarely, sometimes, frequently)

The product of impact and frequency yields a priority ranking. A critical problem users face rarely may warrant less urgent attention than a major problem users face constantly.

Severity rating is used in both heuristic evaluation and usability testing reports. It turns a raw list of issues into an actionable remediation plan. The ratings themselves are somewhat subjective — different evaluators may disagree — but the exercise of assigning them forces clearer thinking about which problems matter most.