A small absolute change can be invisible against a large reference
Aliases: absolute change · relative change · Weber fraction
What it is
With a large reference magnitude, a small absolute change can fall below the just noticeable difference; the identical change at a small reference can be obvious. This direct implication of Weber's law warns against representing important state with a fixed increment alone.
Why it happens
Comparison is affected by internal noise and the current intensity scale. At a large reference, one increment is a smaller proportion and harder to separate from background variation; at a small reference it is more likely to cross threshold. Position, text, colour, and motion provide additional channels so a change need not depend on one numeric dimension.
Studying it
Use several reference values with equal and proportional increments, then measure reliable discrimination. For numeric displays distinguish reading and calculation from preattentive visual detection; their thresholds differ. Record lighting, display scale, and typography.
Where it stops holding
Explicit text allows change detection by reading rather than perception, and high-risk contexts should not rely on apparent visibility. If users do not remember the reference, there is nothing to compare. Absolute values remain important for regulatory thresholds and inventory, so proportional display is not a replacement for them.
Applying it
- For large values or long progress bars, use a proportionally visible change or state the increment and update time explicitly.
- Do not hide small but important updates solely in shade or bar length; add text, a badge, or a notification entry.
- Test discovery of low-amplitude updates at actual viewing distance, not only while zoomed into a design tool.
Related
- Same group: B1.08.1 Just noticeable difference is proportional to the reference magnitude · B1.08.3 Progress and numeric changes should be designed proportionally, not only absolutely
- Nearby: B1.07 Power law of practice · B1.04 Hick-Hyman law
- Search terms:
just noticeable difference·relative change·Weber fraction