Monday, July 22, 2019

Absolute or relative values?

In visualization sometimes the simplest choices are the hardest ones to make. My favorite example is whether to show absolute or relative figures. Take this map by the Urban Institute. It displays what would happen if the Affordable Care Act were repealed. It's the graphic chosen to promote this report on the Web and in social media.

Montana (+177%), West Virginia (+176%), and Maine (+165%), would witness the largest increases, but that's because their populations are small (1.0, 1.8, and 1.3 million). Not surprisingly, the largest absolute increases would happen in more populous states: California (+3.8 million insured), Texas (+1.7 million), and Florida (+1.6 million). You can see the data here. These are the top 10 states in absolute terms:

What's the right choice, total counts or relative values? This is always a decision I struggle with, and my answer is often both. On one hand, it's advisable to use adjusted data—percentages, rates—when designing a choropleth map, but is that map alone enough? Why are 83,000 Mainers and 112.000 Montanans represented by a darker color than 3.8 million Californians or 1.6 million Floridians?

Moreover, what's more informative to someone interested in a topic like this, the relative change or the total number of people who would be left without health insurance if the ACA disappeared? I'd choose the latter, but we should never assume that our preferences are representative of a majority of viewers.