Prefill cuts typing but can insert the wrong value
Aliases: profile prefill · guessed defaults · stale prefill
What it is
Smart prefill is the application writing fields from data it already holds: last delivery address, name on the profile, department in the company directory. It cuts keystrokes and also parks a value that may be stale or belong to someone else in a slot about to be submitted. The browser or a password manager filling by field type, without going through the app’s profile, is not this prefill. This entry is only the yield-versus-wrong-value risk of application-side prefill. It is not whether the source must be named beside the field, and not whether the prefilled value can be edited.
Why it happens
Prefill replaces “recall and type” with “check.” Checking is faster than typing, but checking is recognition: a value that looks like theirs is waved through, especially when there are many fields and little time. A stale address, a coworker’s extension who shares a name, an ID number left from buying a ticket for family—all pass recognition as “still me.” A second layer is the halo of a filled default. Characters already in the box lower inspection depth. Errors shift from “not filled” to “filled wrong and submitted.” Empty can be caught by required. A wrong value in a legal format passes validation; harm lands in fulfillment (shipped to the wrong place, invoice titled to the last company). Net effect is less typing time and more silent error. They have to be read as a pair; completion time alone is not the result.
Studying it
Compare an empty form, accurate prefill, and prefill mixed with stale or other-person records. Do not instruct people to inspect prefill carefully.
Independent variables: prefill present or not, prefill accuracy, whether prefill comes from this person’s last order or from someone else / a stale profile. Dependent variables: completion time, keystrokes, silent errors in the submit (wrong ship-to, wrong title), whether people report having checked prefilled items.
Lab participants told to check will overstate inspection quality. Use time pressure or distraction. Do not count browser autofill mismatches as application prefill—the source is different, and so is the error shape (wrong field versus stale profile).
Where it stops holding
A value just chosen on the previous screen (a plan that carries its price) is not a guess; it is a choice just made, and prefill error rate is near zero. Legal statements that must be typed in person (a signature, a restated consent) cannot be prefilled. Domains where the profile is often wrong (addresses of people who move, a name just changed) will see silent errors outweigh saved keystrokes; offer a suggestion list instead of writing into the box. Brand-new users have no profile to prefill; a placeholder name is an invented error.
Applying it
- Prefill only values from this account that this person confirmed recently. Do not write into the box if the value is older than the business window or comes from someone else’s record.
- Treat prefilled items as high-risk checks: show address, ID, and payout account again in a read-only summary before submit, not only in the middle of a long form.
- Report completion time and fulfillment error (wrong shipment, wrong invoice) together. A drop in time alone is not success.
- Verify by prefilling a stale address and counting how many people submit it unchanged. Turn prefill off and see whether silent errors fall. Take one real fulfillment complaint and trace whether an unchecked prefilled value was the source.