Why adjust the crime rate for seriousness?
The overall level of crime is often measured by the crime rate, the number of violent and property crimes per 100,000 residents. The specific crimes are homicide, rape, aggravated assault, robbery, burglary, larceny-theft, motor vehicle theft, and arson. They’re called “Part 1” crimes.
We believe the seriousness of crimes matters as much to policymakers and the public as the number of crimes. Homicide and petty theft count equally in the crime rate, but their impact is dramatically different.
That’s why our 461-city crime dashboard uses a crime rate adjusted for the seriousness of each type of crime. We don’t try to adjust for other factors, such as the number of workers and visitors. To distinguish our adjusted rate from the standard crime rate, we express it per 10,000 residents, instead of 100,000.
The first step in adjusting a city’s crime rate is to quantify seriousness. We use potential prison sentences set by the California legislature as our measure. For each type of crime, we pick a representative sentence, expressed in months of prison time.
Second, we apply the prison sentences to the crimes committed in the city and calculate the average prison sentence. That number represents the seriousness of the city’s crime.
Third, we compare the city’s seriousness to the California average in our base year (2014). For example, West Hollywood’s seriousness in 2014 was 7% below the state average.
Finally, we adjust the city’s crime rate by that percentage. For example, West Hollywood’s mix of crimes in 2014 was 7% less serious, so we adjust the city’s crime rate down by 7%. Now we can use it in comparisons without worrying about differences in the mix of crimes.
Size of the adjustments
Among California cities, the mix of crimes is three to four times more serious in the most-serious city than it is in the least-serious city. Those differences are reflected in the adjustments. For example, in 2015, the most-serious city’s crime rate is adjusted up by 67%. The least-serious city’s rate is marked down by roughly 40%.
We can see the spread in the chart below. Every point not on the diagonal line represents a city whose crime rate has been adjusted. The points above the diagonal line are cities with more a more serious mix of crime than average. The points below the line are cities with a less serious mix crime.
Effect on West Hollywood comparisons
Some cities that seem comparable to West Hollywood based on the standard crime rate are quite different when we take seriousness into account. For example, the standard crime rates for Desert Hot Springs and West Hollywood were within 1% of each other in 2015. However, the adjusted crime rate was almost 60% higher in Desert Hot Springs. They had seven homicides that year, compared to two in West Hollywood.
San Luis Obispo’s standard crime rate was close to West Hollywood’s but a bit lower (4%). On the other hand, their adjusted crime rate was almost 25% lower, because the majority of their crimes were small-dollar thefts. In West Hollywood, small-dollar thefts made up only a third of crimes.
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