The Definition of Rate

From Wikipedia:

A rate is a special kind of ratio, indicating a relationship between two measurements with different units, such as miles to gallons or cents to pounds. For example, suppose one spends 9 dollars on 2 pounds of candy. The rate $9 / 2 pounds compares the money spent to the number of pounds of candy.(or money)

So the “murder rate” of a city would be the ratio of the total number of murders to another number, like the total population. The murder rate is usually presented as the number of murders per 100,000 residents.

There is a difference between the murder rate and the total number of murders in a city.

The *total number of murders* in Houston was up 13.5 percent in 2006 when compared to 2005:

In 2006, the Houston Police Department recorded 379 homicides as of Dec. 31, a 13.5 percent increase from the 334 homicides recorded in 2005. The 2006 total is the highest since 1994, when 419 homicides were reported in the city.

But the *murder rate* was up 5.57 percent:

Despite another annual increase, residents here are not necessarily at greater risk of becoming a homicide victim. That's because Houston's homicide rate per 100,000 residents rose only incrementally in 2006 — since the city's population is estimated to have surged by more than 148,000 people, due largely to an influx of Hurricane Katrina evacuees.

***

Despite the upward trend, Houston's homicide rate per 100,000 residents hardly changed at all. That number increased from 16.33 in 2005 to 17.24 in 2006.

The FBI says that in 2006, cities with more than 1,000,000 people (that includes Houston) had an average increase in their murder rates of 6.7 percent.

Given the national trends, I just don’t understand how the Katrina evacuees that went to Houston can be blamed entirely for Houston’s higher murder rate:

"We did have a surge in population from a city where the homicide rate is eight times the national average," said Houston Mayor Bill White, referring to New Orleans, hometown of many of the evacuees. "The last 2 1/2 months, we have seen a return to homicide rates typical of the 2000-to-2004 period prior to Katrina."

Yes, more people were murdered in Houston last year. But there were more people in Houston to be murdered. If nearly 200,000 people had come to Houston in a matter of months from anywhere in the world, the total number of murders would have gone up, even if the murder rate had stayed the same.

I am still not sold on the Katrina Criminals Myth.