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Measurement is in the Eye of the Beholder

We are all used to seeing data displayed as counts and percentages. However, as researchers we know that there are many different ways to express data, and each method can paint a different picture.

One of my favorite, most uplifting, data sets looks at mortality data.[1] Back in 2012 the New England Journal of Medicine published an article comparing mortality rates from 1900 to rates from 2010. The graph below, taken from the article, is striking; you can clearly see that infectious diseases like pneumonia, influenza, and tuberculosis caused as many deaths as cancer and heart disease currently cause. But, because the graph reports deaths per 100,000 population, instead of overall number of deaths or percentages, it also provides a plethora of other information.

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Source: Jones, D.S., Podolsky, S.H., Greene J.A., The Burden of Disease and the Changing Task of Medicine. The Burden of Disease and the Changing Task of Medicine, N Engl J Med 2012; 366:2333-233

For instance, the bar on the left is nearly double the height of the bar on the right. What does that mean? It means that if the population in 1900 was the same as the population in 2010, nearly twice as many people were dying in 1900 as die now. Additionally, look closer. The common chorus these days is that heart disease has skyrocketed as a cause of death. But has it really? Compared to 1900, heart disease rates have increased from 137.4 per 100,000 to 192.9 per 100,000. An increase, certainly, but not tripling or doubling. It has, though, gone from causing 10% of mortality to causing one-third.

Cancer rates, on the other hand, have nearly tripled per 100,000 population, perhaps partially as a result of more people living long enough now to get cancer (but that is a story for another blog post).

If we were looking merely at percentages, we’d see that heart disease makes up about 33% of all mortality in 2010 and pneumonia, influenza, and tuberculosis together made up about 33% of all mortality in 1900 but we would NOT see that mortality rates overall have decreased substantially. Relatedly, if we looked only at counts, we would see that fewer people die in 1900 than die now simply because there were far fewer people in 1900 than in 2010. Neither presents a complete picture.

Luckily, the complete picture in the U.S. in 2017 is far better than it was in 1900 – thanks to vaccines you almost certainly won’t die from diphtheria.