Overconfidence in crime statistics doesn’t pay. In a new study, a team of criminologists makes the case that reported crime rates should acknowledge uncertainty in the data. The research demonstrates that rankings of cities as safer or more dangerous — which can influence tourism and tax spending — can be highly misleading.
“If you look at crime rates from year to year and you see a change, there’s a fundamental ambiguity in whether that change is caused by a real change in crime, a change in reporting or some of both,” says criminologist Robert Brame of the University of North Carolina at Charlotte, a coauthor of the new study. “Our position is we should own that. There’s ambiguity here and we should learn to deal with it.”
To get a sense of that ambiguity, Brame and his colleagues calculated the wiggle room in burglary data for the 10 biggest cities in North Carolina. Based on state population estimates and the residential burglaries reported by police departments, the standard simple calculation suggests that in 2009 Wilmington had a higher residential burglary rate than Charlotte, for example. But when the researchers included known uncertainties in the numbers, Charlotte was too close to Wilmington to discern if one really had a lower or higher burglary rate than the other. Ambiguity in the data also meant that the researchers couldn’t tell if the rate of burglaries in Raleigh and Winston-Salem had dropped or risen from 2009 to 2010. Brame, his University of North Carolina colleague Michael Turner and Raymond Paternoster of the University of Maryland in College Park reported their analysis online April 30 at arXiv.org.
A major source of uncertainty lies in how many crimes are actually reported. To investigate this, the researchers used two sources of data: hard numbers of reported crimes and estimates of how many crimes go unreported. The hard numbers came from the FBI’s Uniform Crime Reports, which compiles state data gathered from police departments. For residential burglaries, these numbers are known to be underestimates: If a burglary occurs along with a more serious crime, such as aggravated assault, rape or murder, the incident will be recorded as the higher-ranking offense.