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Debunking BMI - Input Junkie
May 14th, 2009
08:59 am


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Debunking BMI

There doesn't seem to be a single quote that neatly ties together the history of BMI and the reasons it doesn't measure anything significant, but I do recommend the essay.

Link thanks to a comment at Kate Harding's Shapely Prose. I would be surprised if there's a top-level post about it in the forseeable future.

Addendum: How much good information about a group can you get from individual measurements that don't tell you anything important about the individuals?

(2 comments | Leave a comment)

[User Picture]
Date:May 14th, 2009 01:36 pm (UTC)
The general argument is sound, but he gets annoyingly stupid about the math. First there's the whole "why that weird 703 number?" bit. At least he diagnoses that correctly: it's because in America we use a weird system whose numbers don't fit together well, that isn't used by most of the rest of the world. The he says, "why square the height? Why use kilograms and meters instead of, say, grams and centimeters?" The answers, of course, are that you square the height to make its contribution to the total number larger and more like reality (because a fit man of 6'3" really *does* weigh considerably more than a fit man of 5'9", despite only an 8% height difference, for reasons I'm pretty sure are related to the cube-square law) and you could use whatever units you wanted, but you'd just need to figure out the average / underweight / overweight ranges accordingly. There's nothing magical about a normal value of 22, it's just the number that works with this formula.

Then he calls it "bad statistics", right *after* pointing out that it was only ever meant as a statistical tool. He's right that it's a terrible individual diagnostic, but it's just fine statistics, and is still used that way. (As a Penn undergrad, from 1986-89 I worked for Prof R. Arlen Price who used it as intended on studies of the heritability of obesity.)

His major points are all correct; I just hate to see someone goof up a good rant with bad math.
[User Picture]
Date:May 16th, 2009 04:14 pm (UTC)
How about:

Dr David Haslam, the clinical director of Britain's National Obesity Forum was more blatant in a statement he made to the Daily Telegraph newspaper: "It's now widely accepted that the BMI is useless for assessing the healthy weight of individuals."

Unfortunately, changing the emphasis on BMI would require some bureaucrat to admit that he'd made a mistake. Not likely.

I wonder how the Caltech version would work. index = (weight) * (waist) / (height**2) In English units, "ideal" is an index of 1 for college age men.
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