Correlation is Not Causation

Watching David Brooks and John Tierney both race to write the same column extolling the virtues of obesity and mocking liberals for denying themselves cheeseburgers was pretty funny. Did no one warn David that Tierney got there first? Does David not even read his conservative competitor? Seems that the Times token righties need to coordinate a bit better.

But it was also sad to watch two supposedly powerful conservative minds use some of the most read newspaper real estate in the world to misinform their readers in the exact same way and in service on the exact same agenda. So let's get something straight: the study did not tell you to get fat. It did not tell you to get a little fat. It did not, in fact, tell you to do anything at all. We're dealing with observational data that's widely available and the authors are trying to divine a connection between weight and mortality rates from it. We're also focusing on the rate of death, rather than disability and disease (as the Times article on the study -- though not its op-eds -- notes, the connection between excess weight and diabetes, high blood pressure, and high cholesterol in undeniable). And what, exactly, did the study find?

Well, folks whose BMI rests in the normal category provided a baseline mortality, i.e, their death rate is considered the "normal" death rate. Folks in the overweight, though not obese nor extremely obese categories, die less than their "normal" counterparts. The obese, the severely obese, and the underweight die more than everyone else. Why?

First you've got to look at the basic measure used, the BMI. The central mistake of Tierney and Brooks is believing that the BMI is a body fat test. It's not. Instead, it's a purely mathematical calculation that takes your weight, divides it by your height in inches squared, and then multiplies the product by 703. So to give an example, my weight is 188. I'm about 72 inches tall (calculate yours here). So my BMI is 25.5, a shade into the overweight category (which goes from 25-29). But am I overweight?

Now things get interesting. I've lifted weights regularly since my first year of high school, making me a fair bit stronger than most folks. I also have a body fat scale, a relic from when I lost 50 pounds as a high school sophomore (I was a fat kid). My body fat is regularly around 15-16%, which'd put me in the "fitness" category, a good 10% under obesity and 3% under the beginning of "average". So I'm not overweight given my body's composition, but I am a bit overweight if judged via my height-to-weight ratio. To restate, I'm not overweight, but I am if the BMI is used.

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