When the real estate website Movoto declared Augusta to be the “#1 most dangerous place in Maine” this February, was that fair? An article in the Kennebec Journal listed a number of sensible complaints about this label. In this post, I’d like to develop one thread of criticism a bit further: crime rates in Augusta may appear unfairly large simply because Augusta is a city.

Movoto didn’t do any original research to come up with its ranking; it relied on the FBI’s Crime in the United States report for 2013, to which more than 18,000 law enforcement agencies contributed counts of crimes observed by police. The FBI measures crime as a rate, the number of observed crimes per 100,000 residents. The chart you see here draws from the FBI’s own data to show that the areas of Maine with the highest crime rates are apparently our state’s largest cities, including Augusta.

This trend does not always match other observations, however, and that’s because the FBI’s data is far from perfect. Based on interviews with young people, social scientists Jay Greene and Greg Forster conclude that young people living in cities fight, engage in petty theft, use drugs and drink alcohol no more often than students who live in suburbs. According to criminologists Barry Ruback and Kim Ménard, rape crisis centers report a higher rate of sexual assault cases in rural areas than in cities.

Yet the arrest rate for these sorts of offenses is higher in cities. Why?  Police cars more regularly patrol densely-packed areas in cities and are therefore more likely to see crime when it occurs. When young people commit crimes in suburban or rural areas, few or no police see it because those police are spread out over large areas. When young people commit crimes in cities, they get arrested.

In addition, many people commute to cities during the day to work, study or obtain social services, as Michael Shepherd points out in his article. This trend increases the number of people in cities in the hours when people are most active, and simultaneously decreases the number of people in suburban or rural areas. When some of these suburban or rural people visit cities, they commit crimes. Now, crime rates are fractions, with the number of crimes as the numerator and the number of residents as the denominator. The crimes visitors commit are assigned to cities, but those same visitors are not counted as residents of cities when calculating the rate of crime. Instead, they are counted as residents of the rural or suburban places they only sleep in.

Put it all together and crime rates will appear larger inside cities and smaller outside cities.

For reasons like these, the FBI declares it unfair to rank cities against small towns using its data. For websites desperate to attract readers and advertisers, the headlines may be just too juicy to resist.