Monday, July 04, 2005

Whence BMI?

With all the current controversy about body mass index (BMI) and health, BrooklynDodger wondered where the index came from. What's the theoretical or biological basis of that particular formula?

The source of BMI appears to be a treatise by Adolphe Quetelet, a Belgian scientist, in 1835.

From the secondary sources available, Quetelet was intent on finding biometric measurements to be normally distributed. BrooklynDodger is just guessing that the math of the normal distribution was being worked out about then, and provided impetus to fitting to that curve.

Not much of interest in biology or social science is normally distributed, or is, symmetric around a median which is also the mean and mode.

The cut points for under, normal, over-weight and obese are independent of Quetelet's identification of a formula. When the magic numbers of 25 [top of normal] and 30 [bottom of obese] are such round numbers, you can assume there's some assumption behind them, rather than a statistical process.

[By the way, did you ever wonder where 98.6 F for normal temperature came from? It's 37 Celsius. Did you ever wonder about where the .357 magnum came from? It's 9 mm. Why is 11 lbs a force limit for ergonomics? It's 5 kg. Numerology has force.]

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