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Areal unit problem

As explained in D3.js in Action, Third Edition

The areal unit problem is the distortion that occurs when statistical results depend on how geographic areas are bounded or aggregated.

The areal unit problem is distortion caused by geographic boundaries or aggregation choices that make areas overrepresent their data values. In a choropleth map, a large region can attract more visual attention than a smaller region with similar data. section 13.2.3

Russia and Japan on a population map

Russia’s territory is much larger than Japan’s, although their populations may be similar. Applying the same color treatment to both can make Russia appear more important because its larger shape draws more attention. A tooltip can supplement color with the country name and exact value. section 13.2.3

How D3.js in Action, Third Edition covers it

How deep it goes
Defines the distortion and illustrates it with a geographic example, but does not develop formal correction methods.
Examples
Uses a Nobel-laureate choropleth map, Russia and Japan, and country tooltips as concrete illustrations.
What you can do after
Evaluate choropleth maps for area-driven visual bias and add tooltips that expose exact values.
How it connects
Connects choropleth maps, color scales, geographic boundaries, data aggregation, and tooltips.
Cover of D3.js in Action, Third Edition
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This explainer was generated by Manning's knowledge graph from the text of D3.js in Action, Third Edition. The section links point to the parts of the book it draws on.