Every saccade carries a re-acquisition cost
Aliases: re-fixation · landing error · gaze travel
What it is
On a checkout page the unit price lives in the card, shipping inside a collapsed Delivery block, the total pinned to the bottom. Each comparison sends the eyes on a jump, and after landing the new place has to be re-identified as “the number I meant to read”. Saccade cost is not the one or two hundred milliseconds of flight. It is the re-aim after landing: the wrong row, the neighbour’s digits, the return jump to check.
One jump is tolerable. Checkout asks unit price, discount, shipping and total to be reconciled across several rounds. Jump count then multiplies both time and the chance of reading the wrong figure.
Why it happens
A saccade unloads the fovea from A onto B. Visual input is almost unusable in flight; after landing a correction is still due, because saccades slightly overshoot. If B has structurally similar neighbours — a column of aligned money — the landing easily slips to the next one, and the number read belongs to someone else. Ruling that doubt out costs a jump back to A. The round trip is the bill.
Larger amplitude, larger landing error, longer re-aim. That is why “related but on two screens” is dearer than “related but one gap apart”: not because the gap number is bigger, but because there is more room for a failed landing in between.
Studying it
Use a comparison-reading task: “are these two numbers the same?” or “does the total equal these lines?”, with eye tracking.
Independent variables: on-screen distance between the ends, similar distractors on the path, whether both ends share a viewport. Dependent variables: number of round-trip saccades, proportion of landings on the wrong object, judgement accuracy, total time.
Without a tracker, alternate-mask the two ends so only one is visible at a time, and use judgement time and errors as a proxy for re-acquisition. Keep the numbers short so the task does not become a memory-span test — the object of measurement is locating, not reciting.
Where it stops holding
A location in long-term spatial memory (a desktop menu bar, a phone status bar) can be jumped to with almost no re-aim, because the landing is locked by expectation. Experts on a panel they use daily grow that expectation and the cost falls; their numbers must not be used to accept a UI for new users. Motion that tows the eyes changes the cost structure, but that is attentional capture, another topic. Screen readers walk source order; there is no saccade, and the cost becomes how much unrelated speech sits in between.
Applying it
- Put the numbers needed for one judgement in a single fixation cluster: unit price, quantity and line total should not live at the card’s head, tail and the page footer.
- If money columns must split, do not park identically formatted figures around the likely landing.
- Do not put a hint in the left rail and the field at the top of the right — every keystroke then buys a jump.
- How to check: give a reconciliation task and, from a recording, count how many times the head or the eyes round-trip between the two sites. Anything above the minimum trips the task needs is re-acquisition charging.