The Atlantic ran a story under the headline, "Is Costco Making us Fat?"

It talks about the rise of obesity in the U.S., and proposes that cheaper food might be the main culprit. What I like about it is the description of how they used multiple regression to decide which of many factors has the strongest relationship to obesity.

Along with three other researchers, Charles Courtemanche, an assistant professor of economics at Georgia State University, analyzed a number of these theories for a recent study.

Courtemanche realized that a lot of past studies came to conclusions like, "the increasing popularity of driving to work is correlated with the rise of obesity." But he wasn't sure what, specifically, was having the greatest effect on obesity's rise.

He gathered 27 things he thought might be contributing to obesity—everything from working hours to exercise trends to gas prices—and put them in what he calls a "statistical horserace." Essentially, he took all the studies that associated some factor with obesity and saw what would happen if he held everything else constant, like demographic or economic changes. (For example, he told me, "If you're just looking at cigarette prices, was it really cigarette prices, or is it just that they're related to food prices and gas prices? It's controlling for everything together.)

Only two of the factors ended up being meaningful drivers of obesity: 1) the proliferation of restaurants and 2) the rise of warehouse clubs, like Costco, and super-centers, like those Walmarts that have grocery stores in them.

Of course, it's fascinating to read that the increasing prevalence of warehouse clubs and restaurants is associated with increasing obesity, controlling for so many other factors.

But despite The Atlantic's headline, these data can't support their causal claim. All the variables are measured. And even though they analyzed (and held constant) 27 things, there could still be that 28th, unmeasured variable out there….