What 99% Accurate Really Means on a Body Composition Scale

Episode Summary

A client sent Mikki a link to a very flash Withings bioelectrical impedance scale — NZ$819, marketed as 99% as accurate as a DEXA scan — and asked the obvious question: is it worth the money? In this solo episode Mikki goes and reads the validation document behind that claim, and finds the problem isn't really the scale at all. It's a statistical sleight of hand that turns up constantly in health tech marketing: reporting a correlation and calling it accuracy.

Mikki explains the difference in plain terms, with a worked example where a scale reads exactly half the true value every single time and still produces a perfect correlation of 1.0. She then sets the company's own document against three pieces of independent research — a January 2026 systematic review of BIA against a four-compartment model, Siedler and colleagues' 2023 comparison of fifteen BIA devices, and a June 2026 study putting the InBody H20N up against DEXA — to show what honest reporting of a body composition device actually looks like. She finishes with the four readouts on these scales that aren't worth your attention, a practical protocol for getting genuine information out of the one you already own, and four questions worth asking of any health tech accuracy claim you meet in the next few years.

Key Topics

Correlation is not accuracy. The 99% claim rests on a correlation of 0.99 for total fat mass across 80 people. Mikki accepts the correlation is real, and shows with a worked example why it says nothing about whether any individual reading is right.What you actually want is agreement. Mean difference and limits of agreement, via a Bland-Altman analysis — the standard approach since the mid-1980s, and one the company's document didn't use.What the validation document left out: mean difference from DEXA, limits of agreement, individual prediction error, the age and sex breakdown, the body fat range, whether hydration and fasting were standardised, and whether the validation sample was independent of the data used to build the algorithm. Mikki could find no peer-reviewed publication of it, and notes total body water was validated against a different device entirely.Better hardware doesn't settle the evidence question. Mikki is clear the Withings unit is well built — eight electrodes including a handheld bar, multiple frequencies, current through arms, trunk and legs rather than up one leg and down the other. That is a separate question from whether the accuracy claim stan