It can be tempting to use crime data to compare cities and identify trends. You might see lists on the internet that shout out this or that statistic about the safest or most dangerous cities in America.

But reporting practices can influence perceptions of crime as much as actual crime trends do. And decisions about where to draw a city’s borders and which offenses to count as official crimes can matter as much as how many crimes were reported in a given year.

Take the cases of St. Louis, often considered the nation’s murder capital, and South Bend, Ind., whose crime rate has been cited to criticize its mayor, Pete Buttigieg.