Glenn Reynolds has posted a defence of Lott by Benjamin Zycher. Lindgren has a reply on the same page that should definitely be read by anyone interested in Lott and the survey—there are some very interesting details in there about how Mustard’s and Gross’s stories changed in ways that made their accounts more favourable to Lott.

While Zycher repeatedly accuses Lindgren of incompetence, he only bothers to offer one piece of evidence in support of his claims:

Any undergraduate student receiving a B or better in introductory Econometrics would be able to pick the Ayres/ Donohue work apart. This is for a number of reasons, the most fundamental of which is—and this is an error more appropriate for freshman Statistics 1—that their own interpretation of their estimated coefficients is simply wrong. They discuss two variables purporting to measure the effect of concealed-carry laws, but then fail to understand that it is the joint effect of the two variables, rather than merely one of them, that represents the estimated effect in the model.

Ayres and Donohue have extensive discussions on the interpretation of the two variables in their paper. Those discussions appear on pages 1220–1222, pages 1264–1268 and pages 1277–1280. For example (page 1277, my emphasis):

To calculate the five-year impact of the shall-issue law under the hybrid specification it is necessary to add together the impacts of the intercept and trend terms for individual years and then sum the yearly impacts.

Or page 1264 (my emphasis):

according to the hybrid model, in the year after passage the main effect of the shall-issue law is a 6.7% increase in violent crime, which is dampened by the 2% drop associated with the negative trend variable, for a net effect of 4.7% higher crime. After three and a half years, the conflicting effects cancel out at which point crime begins to fall.

I don’t understand how Zycher managed to miss all of this.


  1. #1 dsquared
    October 2, 2003

    To be honest, the whole “hybrid” approach seems pretty Heath Robinson to me. It’s a really broken way of trying to model a regime shift and it seems to me that you have to be pretty insensitive to the assumptions of the linear regression model to use it at all.

    I’ve a post up on CT at the moment about this; I didn’t understand what the hell Zycher was on about. To my mind, the hybrid model is two models, not one, and his talk about the “estimated effect” is confusing as hell.

  2. #2 Seb
    October 2, 2003

    “I don’t understand how Zycher managed to miss all of this.”

    Oh, don’t be so kind! 😉

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