From: oleary@cbnewsh.cb.att.com (brian.m.leary)
Subject: Re: Kennesaw, 1986: Burglary Still Down 85%
Organization: AT&T
Date: Sat, 23 Jan 1993 05:46:52 GMT
Summary: Isn't the UCR data collected from local law enforcement?

In article <LAMBERT.93Jan21011512@nankeen.spectrum.cs.unsw.oz.au>, lambert@spectrum.cs.unsw.oz.au (Tim Lambert) writes:
> >>>>> On Tue, 12 Jan 1993 16:36:35 GMT, oleary@cbnewsh.cb.att.com (brian.m.leary) said:
> 
> > (Tim Lambert) writes:
> >> (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.  
> 
> > Now just what is sacred about this 10 year period? 
> 
> You can always draw a straight line through two points.  If you want
> to show that there was a statistically significant effect you need as
> much data as possible.
> 

This is not correct.  You need only enough data to satisfy the 
requirements of your model.

> > Why not use the 8 six month periods immediately preceding and the 8
> > following the passage of the law in mid-March 1982?
> 
> Since this 8 years long, this is only a little worse than 10 years,
> provided you choose this as the basis of your comparison *BEFORE*
> collecting data.  If you have ten years worth of data and you only use
> eight years of it, you are being very, very naughty.
> 

Not if you have reason to believe that the effect of the intervention
will decrease with time. 

> > And what about the 89% decrease reported
> > when the seven month period immediately following the law was compared
> > to the same period the year before? (1)
> 
> In Kleck's paper in "Social
> Problems" v35p15,  he states there were five reported residential
> burglaries in the seven months after the law, while there were 45 in
> the corresponding seven months of the preceeding year, an 89% decrease.
> 
> From the "Criminology" v29p541 paper by McDowall et al (which
> provides monthly totals) I computed the total number of burglaries in
> the seven months after the law (23) and for the seven corresponding
> months of the previous year (37).  Something is wrong here.  It's
> possible that 5 out of 23 burglaries after the law were residential
> but it sure as hell ain't possible for 45 out of 37 burglaries before
> to be residential.  One dataset must be wrong.
> 
> McDowall et al's data come from the FBI's Uniform Crime Reports.
> Kleck's data comes from a telephone conversation somebody else had
> with the Kennesaw police chief.
> 
> I think the UCR data is likely to more accurate.
> 

And where does the data in the UCR come from?  From
the local police department(1).  Hmmmm.  Maybe the FBI made
a transcription error.  (By the way, not all Kleck's data
comes from Benenson's phone call.)

> > Because the big jump in burglaries ocuured in 1986 and without it
> > the 10 year study you cite would have had a different outcome? (2)
> 
> Nope.  The authors repeated the calculation excluding 86 and found
> that there was still no significant effect.
> 

Well their parameter measuring the effect of the intervention changed
sign, but as you say it was a small change.

Of course if they excluded 86 and corrected for the 70% population increase
they would probably have come to a different conclusion.
And, of course, used residential burglaries, not total.

Brian Leary

(1) Rossi, Under the Gun, page 153
Newsgroups: talk.politics.guns