Mark Duggan's More Guns, More Crime Paper's Fundamental Flaws
Skip Johnson, a vice president for Guns&Ammo's and Handguns Magazine's parent company Primedia, told us that between 5 and 20 percent of Guns&Ammo's national sales in a particular year were purchases by his company to meet its guaranteed sales to advertisers. These copies were given away for free to dentists' and doctors' offices. Because the purchases were meant to offset any unexpected national declines in sales, Johnson said that own purchases were very selective and produced very large swings in a relatively small number of counties. More importantly, while a precise breakdown of how these free samples are counted towards the sales in different counties is not available, these self-purchases were apparently related to factors that helped explain why people might purchase guns, and these factors included changing crime rates. Johnson indicated that the issue of self-purchases is particularly important for Guns&Ammo because the magazine had declining sales over part of this period. Handguns Magazine was much newer and experienced appreciable growth.
Duggan (2000, p. 1110): Duggan makes the adjustment for the standard errors in column 2 of table 12. Murder and violent crime show statistically significant drops after the adjustment, but Duggan knows that there are also typos for his rape and assault results. Simply divide the coefficients for rape (=-.052/.0232) and assault (=-.0699/.0277) and you will see that they have t's greater than 2. Thus for all the violent crime categories but robbery the adjustment does not change the conclusion. In addition, there is the issue of looking at before and after averages versus before and after trends, with the symmetry in the changes in trends before and after the before and after averages do not show a big change even though the change in trends is very big, especially for robbery.
About half of his violent crime rate estimates show statistically significant drops in violent crime from right-to-carry laws and none of his results show a statistically significant increase.