Fitts's Law in Modern Touch and Pointer Ergonomics

Formulated by psychologist Paul Fitts in 1954, Fitts's Law states that the time required to rapidly move to a target area is a function of the ratio between the distance to the target ($D$) and the width of the target ($W$):

$$MT = a + b \log_2 \left( \frac{2D}{W} \right)$$

Tactical Implications for UI Designers

1. Target Size: Interactive elements (buttons, inputs, icons) must meet minimum ergonomic thresholds. Both Apple Human Interface Guidelines and WCAG 2.2 specify a minimum touch target of 44x44 points (or 48x48 dp in Android Material Design).
2. Distance and Thumb Zones: On mobile devices, primary call-to-action buttons belong at the bottom within natural thumb reach, not tucked into top corners.
3. Infinite Edges on Desktop: On desktop operating systems, screen edges and corners provide 'infinite depth' because the mouse pointer cannot overshoot the physical boundary of the display (e.g., macOS global menu bar and Windows Start button).