Lies, Damned Lies and Statistics: The BMI

And here's another piece in my series for SwiftEconomics on economic fallacies. This one goes outside the "economic" fallacies and take on the good ole BMI:
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Next in Lies, Damned Lies and Statistics Series: Part 10: A College Degree is Worth One Million Dollars
Previous in Lies, Damned Lies and Statistics Series: Part 8: Women Do All the Work but Men Keep All the Money
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Some fallacies aren’t simply a one-and-done stat, but an entire methodology. I cannot for the life of me think of a dumber example of this than the Body Mass Index (BMI).

The BMI was invented between 1830 and 1850 (wasn’t the germ theory of disease still a “theory” back then?) by the Belgian polymath Adolphe Quetelet. It breaks people down into the following categories:

– Underweight
– Normal Weight
– Overweight
– Obesity Class 1
– Obesity Class 2
– Morbidly Obese

OK, so far, so good. Graphically, it looks like this:

And mathematically, it’s all based on the following simple equation:

OK, now we have a problem. As you can see, there is absolutely nothing about muscle composition, body type of either gender (although some more advanced models do take this into account). Regardless, some nerd who’s never picked up a dumbbell in his life is not healthier than a body builder, which this equation would lead you to believe.
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Let’s take a look at some examples using BMI-Calculator.net to do our calculations. We’ll start with professional athletes, using ESPN.com’s player profiles.

– Super Bowl winning quarterback Aaron Rodgers: 6’2″ 225 pounds, BMI: 28.9. Prognosis: Overweight

– Freak of nature Lebron James: 6’8″ 250 pounds, BMI: 27.5. Prognosis: Overweight
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– The big but dominant pitcher C.C. Sabathia: 6’7″ 290 pounds, 32.67. Prognosis: Obese, Class 1

– The tank of a running back Michael Turner: 5’7″ 247 pounds, BMI: 35.44. Prognosis: Obese, Class 2

– Speedy point guard Deron Williams: 6’3″ 209 pounds, BMI: 26.12. Prognosis: Overweight

– Elite NFL lineman, and a very big man for sure, Carl Nicks: 6’5″ 343 pounds, BMI: 40.7. Prognosis: Morbidly Obese

Lebron James: Fatty

Seriously, Carl Nicks is a very big man, but morbidly obese implies he can barely move. More accurately, there’s hardly anyone this guy can’t move. It’s as if proponents of the BMI didn’t know that muscle weighs more than fat.
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Indeed, if this Wiki answer is correct, Denzel Washington weighs 216 pounds. He’s 6 foot even, so his BMI is 29.29. In other words, he’s overweight and just a shade under being obese. And how about the other way. If this Ask answer is correct, than Angelina Jolie—who, at least to me, appears to be one of the more normal-sized, ridiculously beautiful Hollywood actresses—weighs 120 pounds. She’s 5’8″. BMI is then 18.24, or underweight. (At least Calista Flockhart came in underweight too, that’s a small token in the BMI’s defense.)

Kate Harding put together a slide show of pictures of mostly normal people whom the BMI has judged to be “overweight” or “underweight” or “obese.” It’s worth looking at. And if you actually want science not just ridiculous examples, NPR points out that the BMI is basically completely bogus. They give ten reasons, a small sample:

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1. The person who dreamed up the BMI said explicitly that it could not and should not be used to indicate the level of fatness in an individual.
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The BMI was introduced in the early 19th century by a Belgian named Lambert Adolphe Jacques Quetelet. He was a mathematician, not a physician. He produced the formula to give a quick and easy way to measure the degree of obesity of the general population to assist the government in allocating resources. In other words, it is a 200-year-old hack.

2. It is scientifically nonsensical.

There is no physiological reason to square a person’s height (Quetelet had to square the height to get a formula that matched the overall data. If you can’t fix the data, rig the formula!). Moreover, it ignores waist size, which is a clear indicator of obesity level.

3. It is physiologically wrong.

It makes no allowance for the relative proportions of bone, muscle and fat in the body. But bone is denser than muscle and twice as dense as fat, so a person with strong bones, good muscle tone and low fat will have a high BMI. Thus, athletes and fit, health-conscious movie stars who work out a lot tend to find themselves classified as overweight or even obese.

And on and on and on… You get the idea. Yes, there is such a thing as obesity, and yes it is a serious problem, but no, the BMI is not a good way to measure it. How this stupid measurement of nothing has stuck with us for so long is beyond me.
Photo Credit: BBC and TampaBay.com
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Lies, Damned Lies and Statistics Series
Part 1: A Primer
Part 2: Income Stagnation
Part 3: All Fiat Currencies Fail
Part 4: Iraq War Casualties
Part 5: Female-Male College Gap
Part 6: Male-Female Wage Gap
Part 7: Roger Maris’ Asterisk
Part 8: Women Do All the Work but Men Keep All the Money
Part 9: The BMI
Part 10: A College Degree is Worth One Million Dollars