| Validate a prototype before development | Usability testing (5 users) | Finds ~85% of usability issues with minimal cost |
| Understand information architecture | Card sorting (open) | Reveals users' mental models for content grouping |
| Measure satisfaction at scale | Survey (SUS/UMUX-Lite) | Quantitative data from large sample, benchmarkable |
| Compare two design variants | A/B testing | Statistical evidence for conversion/engagement differences |
| Audit an existing interface | Heuristic evaluation | Fast, expert-driven, catches obvious violations early |
| Understand long-term behavior | Diary study (7-14 days) | Captures real context, habits, and pain points over time |
| Identify drop-off points | Analytics + session recordings | Data-driven insights on where users struggle or abandon |
| Validate navigation structure | Tree testing | Tests findability without visual design influence |
| Understand user context deeply | Contextual inquiry | In-situ observation reveals tacit knowledge and workarounds |