Ezra Klein on Superfreakonomics

He has a review in the Barnes and Noble Review. He does have a large section about that infamous chapter [see this post for links] on global warming, but here's his take on another claim:

Much attention has accrued to global warming section of the book, and we'll get to that. But for my money, the book's worst tendencies are on display in its beginning pages. There, the Freakonomists begin with an analysis meant to encapsulate the general Freakonomics take on life. But the topic here isn't cutesy or trivial, it is literally life and death. According to Levitt and Dubner, all those teary warnings about driving drunk have obscured the greater danger: walking drunk. That's quite a finding, if true. But is it true?

The Freakonomists arrive at this conclusion in a fairly unorthodox manner. They begin with surveys showing that one out of every 140 miles driven is driven by someone who's blood alcohol is above the legal limit. "The average American walks about a half-mile per day outside the home or workplace. There are some 237 million Americans sixteen and older," they continue, "all told, that's 43 billion miles walked each year by people of driving age. If we assume that 1 out of every 140 of those miles are walked drunk -- the same proportion of miles that are driven drunk -- then 307 million miles are walked drunk each year." Match those numbers against each other and drunk walking proves eight times more dangerous than drunk driving. "Friends don't let friends walk drunk," the Freakonomists conclude.

But that's not a data set. That's an assumption. And a few seconds of consideration will reveal its flaws. For instance: People frequently decide to walk instead of drive because they are going to drink that evening, suggesting that no basic equivalence can be drawn. For instance: People frequently decide to walk instead of drive because they are very drunk. For instance: People frequently decide to walk instead of drive because they live in an urban area, and walking is a viable possibility. In other words, there's not only reason to believe that a higher percentage of miles walked are miles walked drunk, but that the levels of drunkenness are not the same, and the environment is not the same. In other words, this is not enough data to prove anything close to equivalence.

This doesn't stop the Freakonomists, though, who conclude that "friends don't let friends walk drunk." In an interview I conducted with them for C-SPAN's Book TV, Levitt emphasized that "if someone holds a gun to your head," you should definitely drive drunk rather than walk drunk.