Use a bar chart instead of a pie chart for subgroup comparisons
For compare tasks in grouped quantitative displays, use bar charts on subgroup values to improve fidelity and mitigate proportion-reading bias for general audiences.
- purpose:select
- basis:empirical
- task:compare
- chart:bar:use
- chart:pie-donut:avoid
- quality:fidelity
- lever:chart-family
- operator:difference
advice
Bar chart for comparisons
Choose a bar chart when readers need to compare values across groups. For example, use bars to compare risk magnitudes by subgroup and avoid a pie chart when the same job would force readers to judge slices instead of aligned lengths.
reason
Why bar charts work for subgroup comparison
Bar charts support direct magnitude comparison because viewers can compare aligned lengths on a common scale. Pie charts emphasize part-to-whole reading instead, which makes subgroup comparison less direct.
Mechanism: Aligned bar lengths make larger-smaller judgments and gap judgments easier than slice comparisons when the task is to compare groups.
Evidence: The paper summarizes that bar charts are good for making comparisons, especially by subgroup, while pie charts are good for judging proportions and carry biases for that purpose (Lipkus, 2007).
context
Use when subgroup comparison is the job
- User Goal: Compare which group has the larger or smaller value.
- Task: Judge differences across categories or subgroups.
- Data: One quantitative value per group or subgroup.
- Chart Setting: A single display for grouped values rather than a whole-share display.
- Audience: Readers who need straightforward magnitude comparison.
- Success Criterion: Readers can identify the larger group and estimate the difference accurately.
exceptions
Do not use when the task is part-to-whole judgment
Break it when: The main task is judging how one part sits within a whole. Why: The source identifies pie charts as good for judging proportions.
costs
Tradeoffs of replacing a pie chart
Sacrifice: You give up the immediate visual emphasis on a whole divided into parts.
Risk: A bar chart can underplay the whole-share relationship when that is the message.
Mitigation: Use the bar chart only when group comparison is primary, and keep part-to-whole displays for whole-share tasks.
mistakes
Common failure mode in subgroup comparison
Mistake: Using a pie chart to compare subgroup risk magnitudes. Why it fails: The display turns a comparison task into slice reading instead of length comparison.
check
How to test the chart choice
Failure Sign: Reviewers talk about slices or shares when they actually need to compare groups.
Quick Check: Compare a bar version and a pie version and see which one lets a reviewer name the larger group and the approximate gap faster.
Stronger Test: Ask reviewers to rank the groups and estimate the difference from each version.
fix
What to change
- Replace the pie with bars for each group.
- Put the group values into one bar chart so readers compare bar lengths rather than slices.
- Reserve pie charts for part-to-whole readings instead of subgroup comparison.