Interaction Design

Apply Fitts's Law — target acquisition time depends on size and distance. Use when sizing and positioning controls, especially for touch. For how many controls to show at once, use `hicks-law`.

Apply Hick's Law — decision time grows with the number of simultaneous choices. Use when a screen offers too many options at once. For how many items survive in memory afterwards, use `millers-law`.

Design search — query input, zero results, refinement, and result presentation. Use when users retrieve rather than browse. For browse structure, use `navigation-patterns`.

Apply Jakob's Law — users expect your product to work like the others they already use. Use when deciding whether to innovate on a familiar pattern. For OS-mandated conventions specifically, use `platform-conventions` (ui-design).

Design a form end to end — field order, grouping, validation, and completion. Use when the artifact is a form. For product-wide error strategy use `error-handling-ux`; for first-run signup use `onboarding-design`.

Apply Miller's Law — chunk information into groups of about four to fit working memory. Use when grouping fields, menu items, or steps. For reducing the number of choices offered, use `hicks-law`.

Apply Tesler's Law — every process has irreducible complexity that someone must absorb. Use when deciding whether the product or the user carries it. For reducing apparent choice, use `hicks-law`.

Apply the Peak-End Rule — a flow is remembered by its most intense moment and its last. Use when designing completion, celebration, or cancellation moments. For sustaining engagement mid-flow, use `zeigarnik-effect`.

Model component behaviour as explicit states, events, and transitions. Use when a component has many interacting states that must be exhaustive. For the feel and feedback of a single interaction, use `micro-interaction-spec`.

Design waiting experiences — spinners, skeletons, optimistic updates, and progressive reveal. Use when content takes time to arrive. For the latency budget itself use `doherty-threshold`; for success confirmation use `feedback-patterns`.

Design gesture interactions for touch and pointer — swipe, drag, long-press, and their discoverability. Use when input is gestural. For OS-standard gestures on iOS and Android, use `platform-conventions` (ui-design).

Apply the Zeigarnik Effect — incomplete tasks stay mentally active. Use when designing progress indicators, saved drafts, and return hooks. For the emotional shape of the ending, use `peak-end-rule`.

Design voice and conversational interfaces — dialog flows, error recovery, and persona. Use when the interface speaks and listens rather than being tapped. For graphical input collection, use `form-design`.

Apply the Doherty Threshold — keep system response under 400ms to preserve user flow. Use when diagnosing perceived slowness or setting a performance budget. For what to show during unavoidable waits, use `loading-states`.

Design error prevention, detection, and recovery across a product — message content, placement, and escape routes. Use when errors span multiple flows. For validation inside a single form, use `form-design`.

Design confirmations, status updates, and notifications that tell users an action registered. Use when the system must acknowledge success or change. For waiting states use `loading-states`; for failures use `error-handling-ux`.

Design the first-run experience — activation path, progressive disclosure, and time to first value. Use for a user's very first session. For the mechanics of the signup form itself, use `form-design`.

Select and design a navigation pattern — tabs, drawer, hierarchy, or hub — matched to product structure and user tasks. Use when choosing how users move between sections. For the underlying content structure, use `information-architecture` (ux-strategy).

Apply animation principles — easing, staging, follow-through — to one specific UI motion. Use when tuning how an animation feels. For product-wide duration and easing tokens use `motion-system` (design-systems); for a full interaction spec use `micro-interaction-spec`.

Apply an emotional resonance lens to a UI that is technically correct but flat, prescribing changes at the copy, motion, and interaction layer. Use when a design tests fine but lands cold. For the polish-perception argument, use `aesthetic-usability` (ui-design).

Specify one micro-interaction completely — trigger, rules, feedback, loops, and modes. Use when handing a single interaction to engineering. For motion craft alone use `animation-principles`; for multi-state components use `state-machine`.

Apply the Serial Position Effect — first and last items in a sequence are recalled best. Use when ordering menus, lists, and steps. For emphasising one item regardless of its position, use `von-restorff-effect` (ui-design).