## Monday, March 04, 2013

### Once Again: On BMI

It seems like I write something about BMI each year (2009, 2010, 2011), save (strangely) last year. The main thing that I keep pointing out is that BMI is:
1. An objective measurement that is
2. Used improperly, which leads to
3. People imbuing tons of social construction into BMI, because
4. Objective numbers (for some reason) are psychologically powerful (probably because they can't be refuted on their own).
Well, we should all know by now that BMI is definitely not a measurement of an individual's health, of fat, or or anything else other than weight/(height^2).

And what is the significance of weight/(height^2)? About as much as the significance of:

which provides the user with the annual average water discharge of India's Ganges River, based on the total upstream watershed area (A), the annual precipitation of the upstream watershed (P), and the % of the upstream watershed that is in the Himalaya mountains. It's actually a highly predictive formula, with an R-squared of 95.5%, which is - for such large-scale modeling - pretty damn good for using only three variables.

However, this formula is next to useless in determining what the flow of the Ganges river is in any one year, in any one season, or during any one day. Why? Because it doesn't actually measure any of those.

And BMI - much like the simple equation I derived for my master's work - does not measure health or fat simply because it doesn't actually measure those things. It is merely a statistical regression equation that is based on a specific population (19th Century Belgian men) and split into arbitrary categories that mapped (during the 1970s or 1980s) onto concepts of health and obesity.

The BBC World Service did an episode on the efficacy of the BMI and a possible new equation for calculating BMI. (The story did a good job of also showing why the new formula is about as useful for giving individual advice as the existing formula.)