Just Do the Math Fallacy

Why You Can’t “Just Do the Math”

It’s good to think about things. It’s good to try to understand the various arguments around issues of public or personal concern. But to come to better understandings of issues we care about, we rely heavily on the expertise and fair-mindedness of those we read, watch, or listen to.

Take Jill. Jill had a “just do the math” moment with the N95 argument she examined. She read a comparison of virus size with a supposed opening in the filter and concluded that the mask could not work. But the note in the bottle failed to mention a couple of other important things.

1. The Virus Travels in Respiratory Particles. Filtration concerns the particles carrying the virus, including droplets and smaller aerosols, rather than the size of an isolated virus alone.
2. Electrostatic Charge. N95 masks have an electrostatic charge that pulls particles toward the fibers, allowing capture of particles smaller than the weave.
3. Brownian Motion. Random motion contributes to capturing small particles. The stated 3 microns is not a simple boundary separating captured particles from uncaptured ones.

Modeling filtration requires the relevant mechanisms and measurements. A simple size comparison cannot establish effectiveness if it treats the filter as a sieve or misidentifies what a measurement means. Jill needs a better model and reliable information. The F of SIFT—Find a better source—directs her toward explanations grounded in testing and relevant expertise, a skill the authors call “reading expertise.”