Enlarging a target and shortening travel have different costs

What it is

Under Fitts's law, increasing effective width and reducing travel distance can both lower pointing difficulty, but they are not interchangeable design moves. A larger target consumes screen real estate and may crowd neighbours; a closer target changes placement and can disrupt hierarchy, muscle memory, or travel to other controls. The choice must account for the whole task, not merely a smaller D/W ratio.

Why it happens

Both changes affect the distance–width ratio, yet act on different interface constraints. Extra width expands the tolerance band, mainly reducing final correction and misses. Moving a target closer shortens the whole movement but redistributes spatial relationships. Making one control larger or closer also changes other controls' distance, visibility, and accidental-activation risk. A local motor benefit can therefore be offset by a system-level layout cost.

Studying it

Create functionally equivalent prototypes that manipulate size and position separately. Measure movement time, misses, visual-search time, neighbouring-action errors, and perceived usability. For frequent tasks, compare cumulative travel and acquisition across the whole action sequence, not just one button. Report whether the change crowded information, reduced label legibility, or introduced navigation steps; those conditions explain the trade-off.

Where it stops holding

Once a target is much larger than endpoint variability, further enlargement usually has little gain. When it is already near the user's attentional or gestural starting point, further movement soon meets hierarchy and consistency costs. Screen size, one-handed posture, assistive technology, and accessibility requirements change which route is feasible. A control should not be moved to a semantically surprising location merely because a lab task is faster.

Applying it

• For frequent controls, map paths from common start points, then separately assess how enlarging the hit area and relocating the control affect surrounding layout.
• First enlarge the real hit area in ways that preserve visual hierarchy; where space is limited, put the most frequent next action near the current action.
• Test both alternatives in a complete task. Check misses, label comprehension, neighbouring controls, and whether people can still predict where the control lives, alongside completion time.

Related

• Same group: B1.01.1 Pointing time is jointly determined by target distance and width · B1.01.2 Distance and width affect time logarithmically, not linearly · B1.01.3 Width is the effective size along the movement direction · B1.01.4 The law describes an average trend, not one operation
• Nearby: B1.02 Fitts's law on touchscreens · B1.03 Infinite width at screen edges
• Search terms: Fitts's law · target size · layout trade-off · target acquisition