Page 52 - Statistics for Dummies

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Part I: Vital Statistics about Statistics
Now compare the number of crimes and the crime rates for 2001 and 2002 in
Table 3-1. In column 2, you see that the number of crimes increased by 2,285
from 2001 to 2002 (11,878,954 – 11,876,669). This represents an increase
of 0.019% (dividing the difference, 2,285, by the number of crimes in 2001,
11,876,669). Note the population size (column 3) also increased from 2001
to 2002, by 2,656,365 people (287,973,924 – 285,317,559), or 0.931% (dividing
this difference by the population size in 2001). However, in column 4, you see
the crime rate decreased from 2001 to 2002 from 4,162.6 (per 100,000 people)
in 2001 to 4,125.0 (per 100,000) in 2002. How did the crime rate decrease?
Although the number of crimes and the number of people both went up, the
number of crimes increased at a slower rate than the increase in population
size did (0.019% compare to 0.931%).
So how should the crime trend be reported? Did crime actually go up or
down from 2001 to 2002? Based on the crime rate — which is a more accurate
gauge — you can conclude that crime decreased during that year. But be
watchful of the politician who wants to show that the incumbent didn’t do his
job; he will be tempted to look at the number of crimes and claim that crime
went up, creating an artificial controversy and resulting in confusion (not to
mention skepticism) on behalf of the voters. (Aren’t election years fun?)
To create an even playing field when measuring how often an event occurs,
you convert each number to a percent by dividing by the total to get what
statisticians call a rate. Rates are usually better than count data because rates
allow you to make fair comparisons when the totals are different.
Untwisting tornado statistics
Which state has the most tornados? It depends on how you look at it. If you
just count the number of tornados in a given year (which is how I’ve seen the
media report it most often), the top state is Texas. But think about it. Texas
is the second biggest state (after Alaska). Yes, Texas is in that part of the U.S.
called “Tornado Alley,” and yes, it gets a lot of tornados, but it also has
a huge surface area for those tornados to land and run.
A more fair comparison, and how meteorologists look at it, is to look at the
number of tornados per 10,000 square miles. Using this statistic (depending
on your source), Florida comes out on top, followed by Oklahoma, Indiana,
Iowa, Kansas, Delaware, Louisiana, Mississippi, and Nebraska, and finally
Texas weighs in at number 10. (Although I’m sure this is one statistic they
are happy to rank low on; as opposed to their AP rankings in NCAA football.)
Other tornado statistics measured and reported include the state with
the highest percentage of killer tornadoes as a percentage of all tornados
(Tennessee); and the total length of tornado paths per 10,000 square miles
(Mississippi). Note each of these statistics is reported appropriately as a rate
(amount per unit).

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