Encode quantitative values with position when reading accuracy matters
For exact quantitative lookup or comparison, use position encoding instead of area encoding on quantitative displays to improve fidelity and mitigate magnitude misjudgment for readers making precise value readings.
- purpose:refine
- basis:empirical
- task:retrieve
- quality:fidelity:use
- lever:encoding
- reading-mode:exact
- channel:position:use
- channel:area:avoid
advice
Change the quantitative channel to position
Encode the critical quantitative field with position when readers need accurate value judgments. For example, move a magnitude from circle area or rectangle area into x or y position, especially when the current readout depends on bubble-chart or treemap-style area judgments.
reason
Use the more accurate perceptual channel
Position supports more accurate quantitative perception than area. Area judgments, including circular and rectangular area, make precise reading harder and can push readers toward wrong magnitude estimates.
Mechanism: Position lets readers judge magnitude more accurately than area, so exact value lookup and comparison become more reliable.
Evidence: Position encoding has the highest accuracy, followed by length, angle and rotation, and then area; circular and rectangular area encodings are especially low in accuracy, which helps explain why bubble charts and treemaps are harder to read (Börner et al., 2019).
context
Use when the quantitative readout must be precise
- User Goal: Read or compare quantitative values accurately.
- Task: Exact lookup or exact comparison.
- Data: Quantitative values encoded visually.
- Chart Setting: A display currently uses, or could use, area to encode magnitude.
- Audience: Readers making precise value readings.
- Success Criterion: Readers judge magnitudes accurately.
exceptions
Do not use when exact quantitative judgment is not the success criterion
Break it when: Precise value reading is not the success criterion. Why: This rule is specifically about perceptual accuracy of quantitative encoding.
costs
Accept the cost of changing the encoding
Sacrifice: Area-based magnitude displays such as circular or rectangular size coding. Risk: The chart form may need to change if it currently depends on area as the main quantitative channel. Mitigation: Keep the same data but remap the critical quantitative field to x or y position.
mistakes
Avoid leaving the critical number in area
Mistake: Keeping the main quantitative readout in circle area or rectangle area when readers need exact values. Why it fails: Area is perceived less accurately than position, so magnitude judgments become harder.
check
Compare the area version to a position version
Failure Sign: Reviewers disagree or hesitate when reading values from size alone. Quick Check: Redraw one key quantitative readout with position instead of area and see whether the value is easier to read. Stronger Test: Ask a reviewer to retrieve or compare the same values from an area version and a position version; keep the position version if answers are more accurate.
fix
Remap the magnitude to position
- Move the critical quantitative field from area into x or y position.
- Rebuild bubble-chart or treemap-style quantity readouts with position-based marks when exact reading matters.
- Keep the data values the same so the main change is the encoding channel.