>>>>> 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.
> 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.
> 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
preceding 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.
> Because the big jump in burglaries occured 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.
Tim