Defines how a design's success will be measured: HEART goals→signals→metrics, SUS/SEQ/NPS instrument selection, UX benchmarking waves, and analytics measurement plans with macro/micro conversions. Use when asked 'how do we measure success', 'what metrics should we track', 'define KPIs for this feature', 'set up HEART', 'should we use NPS', 'benchmark our UX', 'write a measurement plan', or before any launch that will later be judged. NOT for analyzing usability findings (analyze-usability-data), designing A/B tests (use-quantitative-evidence), or measuring AI output quality (write-ai-evals).
