The Problem with ‘Prevalence’

This word may not mean what you think it means

When writing about change over time—how common a situation or condition was, is or will be—it’s easy to tell a falsehood. The mistake boils down to prevalence vs. absolute numbers. Prevalence has a very specific definition in medicine: It’s the proportion of a population who have a condition, not the raw total number. The general definition also involves percentages.

Perhaps you know that. A lot of people don’t. Ask a couple friends for their quick definition of “prevalence,” and you’ll see.

Here’s an example having to do with health, but the logic holds with any data, be it in business, sports, cultural phenomena or whatever.

The prevalence of dementia is expected to rise over the next 25 years.

I’ve seen variations on that theme in many stories. It’s probably not true, and it’s almost surely misleading, and it’s 100% lacking in context. Let’s examine the math, such as we know it.