Calculating

Express anything as a number, a statistic, and it takes on the air of authority. The more apparently precise that number, the more digits after the decimal point, the more substantial it seems to people — even if that very precise figure was arrived at through a speculative and arbitrary set of calculations.

Take, for example, "BMI" — the body mass index. The paper ran a story on obesity yesterday, including a sidebar explaining how to calculate your BMI.

If your BMI is between 18.5 and 24.9, the article said, you are "normal." If your BMI is above 30, you are considered obese. But 18.5 or 30 whats? What are the units here? And how do you arrive at the figure that represents your figure?

To calculate your BMI, first take your weight in pounds and divide it by your height in inches, squared.

Already I'm skeptical — dividing pounds by inches seems like an odd muddying of different measures. And why is your height squared? Height, after all, is a single dimension. I kind of thought that height, by definition, can't be squared. You start squaring things and you're no longer just talking about height, you're talking height x width.

Anyway, take this number, this plum-pudding of inches and pounds, and multiply it by 703. That's your BMI.

What explains the use of 703 as our constant here?

Why not use a good old, woven-into-the-mysteries-of-the-universe constant like pi? One arbitrary constant is as good as another isn't it? So why not use the time-tested ratio of the circumference to the diameter of a circle instead of the area code for northern Virginia?

The number 703 apparently gets plugged in just to make the final range of figures more manageable. Replacing it with pi would give everybody a BMI of less than one. But we can fix that by no longer squaring everybody's height all willy nilly, thus reducing the denominator in our equation.

So, using this new calculation, I find I have an ABMI (adjusted body mass index) of 7.85. Which means …

It doesn't mean anything. Arbitrary nonsense with three digits after the decimal point is still arbitrary nonsense.

We ought to know whether we are skinny or fat without depending on some faux-authoritative bit of quantification. The current enthusiasm for calculating BMI is mostly harmless, but our gullibility for misplaced concreteness is a dangerous thing.