Use Fitts's Law to size and place primary vs secondary mobile actions

WHAT THIS TESTS: This question evaluates whether you can move beyond memorizing Fitts's formula and instead use it as a design heuristic for constrained mobile environments. The interviewer wants to see that you understand movement time is a function of both target size and distance, that the relationship is logarithmic rather than linear, and that fingers are far less precise than mouse cursors. They are also checking if you can hierarchize actions, giving primary buttons preferential spatial treatment over secondary ones.

A GOOD ANSWER COVERS four things in order. First, state Fitts's Law concisely: the time to acquire a target increases with distance and decreases with target width, and because the equation is logarithmic, doubling the distance does not double the time. Second, apply this to mobile touch by noting that fingers require larger targets than cursors, so primary actions should be sized generously, typically 48 density-independent pixels or more, and placed within the natural thumb zone, usually the bottom third of the screen. Third, explain how secondary actions are treated differently: they can be smaller or positioned farther from the thumb's resting position, but they must still respect minimum touch targets to avoid errors. Fourth, mention that screen edges and corners are special cases because they provide infinite effective width in one dimension, making them fast to reach even if they are distant, which is why destructive or frequent actions are often anchored there.

COMMON WRONG ANSWERS include treating the size-distance relationship as linear, suggesting that simply making every button huge is sufficient without considering placement, or applying desktop mouse logic directly to touch. Another red flag is ignoring the two-component model of movement, where an initial fast coarse motion is followed by a slower precise adjustment; this explains why very small targets disproportionately slow users down.

LIKELY FOLLOW-UPS include asking how you would adapt these principles for left-handed users or one-handed versus two-handed holds, how accessibility settings like dynamic type change target sizing, or how you would justify a small secondary target to a product manager who wants every button equally prominent.

ONE CONCRETE EXAMPLE: In a checkout flow, the primary Pay Now button should sit at the bottom center of the screen at 48dp height and full width, placing it inside the thumb zone and maximizing width. A secondary Change Payment Method link can sit above it at 44dp, farther from the thumb's resting position but still tappable. If a Cancel action is needed, it can be anchored to the top-left screen edge, leveraging the edge's infinite width for fast acquisition without risking accidental taps near the primary action.

Source: nngroup.com