Apply the empirical laws of user behavior — Fitts's Law, Hick's Law, Miller's Law, Jakob's Law, Tesler's Law, the Doherty Threshold, Gestalt grouping, serial position, peak-end, Von Restorff, goal-gradient, aesthetic-usability. Use when sizing or placing targets, deciding how many options to show, judging whether an interface is too slow, explaining why a layout groups wrong, debating menu length, ordering lists, designing progress or multi-step flows, or when someone cites a "law" and you need to know whether it actually applies.
