The news from the F.B.I. crime data was alarming: The murder rate rose sharply last year, driven by jumps in several major cities.
Four urban areas — Baltimore, Chicago, Milwaukee and Washington — accounted for about a fifth of the increase in homicides in 2015. Those cities, however, make up only about 1 percent of the nation’s population.
But whether crime is up or down depends on what data is being looked at — and who is doing the looking.
The F.B.I. data showed that violent crime rose about 4 percent last year from 2014, and homicides increased 10.8 percent. Yet crime over all fell in 2015 for the 14th consecutive year.
And the total number of homicides last year was fewer than 20 years ago even as the country’s population increased, criminologists said. There were 19,645 homicides in 1996 in a nation of 265 million; in 2015, there were 15,696 in a population of 321 million.
What that data means, criminologists and police officials said, is that the decline in homicides has been so significant in the last quarter century that sudden increases in the number of killings in just a few cities can skew the entire national picture, even as the country has one of its safest periods on record.