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Bias of the day: immortal time

This is when you do a retrospective study, select cohorts according to exposure, but measure outcomes — usually death, or hospitalization, or something else bad — in a way that guarantees one or more of the cohorts have a period of time when that outcome couldn’t have happened. That’s how you get “immortal”, or “guaranteed” time.

Three classic examples: Courtesy of Bing.

To these three classics we can now add two more, one highly publicized, the other less so, both surprising considering the journals and the supposed peer review they must have gone through:

Cardiologist John Mandrola explains in depth why the LAAO paper, and the way it was spun, is particularly egregious.

Note that this is only a problem in retrospective — or, how they now like to be rebranded, “real-world” — studies. As the most recent cases show, these are not only worthless for informing anyone’s real-world decision, but also contribute to the noise, the chaos, and the general fear-uncertainty-doubt of medicine. A voluntary moratorium would not be out of line.

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