Encode a single risk proportion as a countable unit array

Show the full denominator as equal unit marks and highlight the affected marks directly. For example, use a pictograph with 100 small squares that shows both affected and unaffected people instead of a pie slice when you need to present one risk statistic at a single time point.

Countable unit arrays make the whole population and the affected subset visible at the same time. That gives readers a direct read of both the numerator and the denominator instead of making them infer the whole from an angle or area.

Mechanism: Repeated unit marks turn one abstract proportion into visible affected and unaffected cases, which supports faster reading and more accurate exact-number interpretation.

Evidence: The paper reports that pictographs are more quickly and better comprehended than other graphical formats, visually convey both affected and unaffected cases, and outperform pie graphs for exact-number knowledge when communicating individual statistics (Fagerlin et al., 2011).

Notes: The paper gives its strongest support to pictographs when readers need to think carefully about risk statistics.

• User Goal: Understand one risk or side-effect probability clearly.
• Task: Read the exact affected and unaffected counts, not just the general direction.
• Data: One proportion with a fixed denominator at one time point.
• Chart Setting: A static risk graphic in patient education material or a clinician explanation.
• Audience: Patients making treatment decisions, including many with low numeracy.
• Success Criterion: Readers can quickly recover both the event count and the remainder of the population.

Break it when: The display needs to show how risk changes over time. Why: The paper notes that a pictograph may be more difficult than a line graph for showing change in risk pattern over time.

Sacrifice: You give up some flexibility for showing temporal change. Risk: Forcing the same unit-array format to carry a time pattern can make change harder to see. Mitigation: Switch to a time-series graphic when the message is how risk changes over time rather than the value of one risk at one time point.

Mistake: Show only a pie slice or a single percentage without the visible unaffected denominator. Why it fails: Readers must infer the whole population and can miss the exact relationship between affected and unaffected cases.

Failure Sign: The graphic shows one proportion, but a reviewer cannot directly inspect both affected and unaffected cases. Quick Check: Can a reviewer point to the highlighted affected units and the remaining unaffected units without doing arithmetic? Stronger Test: Compare this version with a pie-slice version and see which one better supports exact readout of both affected and unaffected counts.

• Replace the single slice or abstract probability with equal unit marks for the full denominator.
• Highlight only the affected units while leaving the unaffected units visible.
• If the message is about change over time rather than one time point, replace the unit array with a line graph.