# Gun control math, part 1. (a.k.a., don't predict what you use as explanation...)

OK, I waded into housing controversy last time, and now I want to take a brief dip into one of my favorite, incredibly controversial, policy topics: gun control.

There is a long running policy debate about whether increasing the number of guns in a society impacts crime rates. There is a famous book called “More Guns, Less Crime”, written by a pro-gun activist slash econometrician named John Lott.

Lott’s thesis was that an increase in availability of guns — usually because of “right to carry” laws — causes a reduction in crime. Lott uses regression methods to attempt to justify his conclusion; his prose often emphasizes the mathematical minutiae; it seems that he’s trying to beat the reader over the head with the idea that "math is perfect, you should simply believe the prose!"

By the way, please never, ever, do that. Check the math. Really. And feel great doubt for any article that seems to shroud its answer in a bunch of seemingly impenetrable prose.

It will take me a bunch of (400 word) postings to cover all the math mistakes in the gun control literature. Both sides use math creatively in order to get the answer they want. But here’s the first.

In 2005, the National Research Council reviewed the research on right to carry laws, and more or less threw up its hands saying that neither Lott nor his primary critics had demonstrated whether the laws had any impact on crime.

The NRC study replicated a pretty basic error Lott made. In Lott’s regression equation, he has murder rate on both sides of the equal sign. That is, murder is BOTH predicted by the equation AND in the equation that is doing the prediction.

Having the same variable on both sides of the equation makes it far more likely that the equation will predict the outcome. In other words, the regression looks more significant than it really is. This can lead to a policy prescription that is either “throw up your hands” or, perhaps “proves that more guns equals less crime”.

Based solely on this data, the latter is likely false. The former conclusion seems cowardly.

Please don't assume, however, that the pro-gun control side's math is as pure as the driven snow. Stay tuned for the beginning of the errors on the other side…

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