A full body MRI earns you a year of smoking

Alternative titles:

… earns you a high-risk pregnancy

… earns you an ascent of Matterhorn

… earns you 10,000 km on a motorcycle

… earns you two base jumps

jumps … earns you a day on the frontline in Ukraine

These are all about equivalent to the risk of one year of smoking.

I’m skeptical of submitting asymptomatic people to medical tests. Almost any time I look into the evidentiary power of medical tests, I’m struck by how much work is performed by the base rate. The tests only work because we perform them on people who appear sick, i.e. have a higher probability of actually being sick in the first place. If we would perform them on seemingly-healthy people we would get nonsensical results.1 Note that this is not a criticism of the tests. They are optimised for the thing they need to do.

Thus it was with interest I read Scott Alexander’s breakdown of the benefits and costs of a routine full-body mri as a way of screening for cancer. However, I felt like the conclusions weren’t put into an understandable context. Here’s my attempt at doing so.

First, a quick recap of the main points of the article. I’ll ignore the exorbitant financial costs of us healthcare, and focus on the benefit and cost in terms of health. This is measured in quality-adjusted life years, or qaly s. Of the hypothetical thousand people who get scanned, the estimation is that

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