Deltoid

brian.m.leary said:

The residential burglary rate in Kennesaw, Georgia dropped sharply after a city ordinance requiring heads of household to keep at least one firearm in their homes was passed. The law passed early in 1982. In 1986 the rate was still down 85% compared to 1981. (1)

This statistic is essentially meaningless. If the crime rate fluctuates, then by picking the right two years to compare, you can get any result you want. To make a credible case, you need to provide data for at least the ten years 77-86. I haven't seen Kleck's book, but in his "Social Problems" paper, he uses figures for residential burglary that are contradicted by the official UCR data.

What about that study that shows no drop in burglary in Kennesaw? Well strangely, the authors used the total burglary rate not the more relevant residential burglary rate.

Does the UCR data break burglaries down into residential and non-residential ones? If not, then in the absence of reliable data on residential burglaries, it is best to use total burglaries. Since this includes residential burglaries, it would be surprising if a reduction in the residential rate did not appear as a reduction in the total rate.

In another strange error(*), the authors did not account for Kennesaw's seventy percent population increase over the period. (2)

The population would have to increased by over 500% between 81 and 85 for there to be an 85% drop in the population adjusted rate, and no reduction in the actual number of burglaries.

The population increase would occur gradually over the period and could not mask an abrupt change in the burglary rate, so dividing the number of burglaries by the population will not change the result of the interrupted time series analysis: Here are the burglary rates 76-86, assuming a population growth of 70%, normalized to the 76 population value. Each x represents 5 burglaries.

xx             xx             xx
xx       xx x  xx x  x        xx
xx xx xx xx xx xx xx xx xx xx xx
xx xx xx xx xx xx xx xx xx xx xx
76 77 78 79 80 81 82 83 84 85 86
                  ^
                  Law        

The rate does not appear to be lower after the law.
Average 76-81 is 29
Average 82-86 is 26
The average after is a little lower. To test the significance of this, I use Student's t-test. I compute t=-0.13 with 9 df, p=0.51, that is, 51% of the time chance will produce a change this large. This obviously not significant.

In any case, a population increase of 70% weakens any evidence for a deterrent effect. Such rapid growth means Kennesaw changed significantly during the period in question, so even if there was a significant reduction in burglaries, we don't know whether one of the other changes in Kennesaw caused it.