Smaller targets require more submovements

What it is

Once the same initial impulse has sent a cursor or finger near the target, how narrow the tolerance around that target is determines how hard it is to be judged "hit": the smaller the target, the more likely the same amount of impulse deviation gets flagged as a miss by that tighter tolerance, requiring more rounds of submovements to progressively narrow the error until it falls within that smaller range. Target size and the number of submovements needed are directly, causally linked — not two things that just happen to co-occur.

Why it happens

The impulse phase carries a certain amount of output variability, and the size of that variability is relatively fixed — it doesn't automatically shrink just because the target got smaller. When a target is large enough, the probability of a single impulse landing within it is already high, so the task can be finished with very few or even zero submovements. As the target shrinks, a larger share of that same impulse variability gets filtered out by the tighter acceptance criterion, and subsequent submovements are needed to progressively drive the landing point into that narrower range. Each submovement itself also has a precision ceiling of its own, so a smaller target may not just need one extra step — it may need several, each compressing the error down by another increment until it meets the requirement.

Studying it

A common way to verify this relationship systematically varies target width while holding other conditions (distance, display method) constant, and measures average submovement count across width conditions: if submovement count rises monotonically as target width shrinks, this supports the conclusion that target size directly determines the number of correction rounds needed. Such studies are typically viewed alongside overall completion-time data, since a rise in submovement count is itself the mechanism-level explanation for the familiar phenomenon that smaller targets take longer to complete — not just an empirical rule on its own.

Where it stops holding

This describes how submovement count changes with target width — it doesn't involve a specific quantitative formula or logarithmic relationship, which belongs to a different layer that models width, distance, and time together. Also, how sensitive submovement count is to target width differs by input device — some devices (a stylus, a precision mouse) already have smaller impulse-phase output variability, so the same reduction in target width produces a gentler rise in submovement count than on a device with larger variability (coarse finger touch).

Applying it

• For small targets that appear in an interface (icons in a dense list, a fine adjustment handle), expect users to go through several rounds of submovements before hitting them, and that this process itself lengthens completion time; if the operation is speed-sensitive, prioritize enlarging the target's effective hit area rather than relying only on visual cues or instructional text to compensate.
• For small targets that genuinely can't be enlarged (constrained by layout density), consider providing gentle snapping or deceleration assistance as the cursor or contact point nears the target — effectively shrinking the number of submovements still needed within that region, compensating for the target's own size limitation.
• How to check: measure average submovement count and total completion time on several targets of different widths, and plot both against width. If adjusting a specific target's width produces a matching drop in both submovement count and completion time, that target was indeed sitting in a regime where its small size was forcing multiple rounds of correction.

Related

• Same group: A8.08.1 The initial impulse phase covers most of the distance quickly · A8.08.2 The correction phase closes in on the target with small submovements · A8.08.3 The number of submovements determines total movement time · A8.08.5 Undershoot and overshoot of the initial impulse carry different costs
• Nearby: A8.09 Speed-Accuracy Tradeoff · A8.06 Open-Loop Control
• Search terms: target width effect · submovement count · index of difficulty