Is murder rising or falling nationwide this year? It’s a simple question but surprisingly hard to answer.

Every year, nearly 20,000 state and local law enforcement agencies collect data on a variety of crimes. The F.B.I. compiles the data, and at the end of September — of the following year — it releases the Crime in the United States report, the definitive accounting of crime nationally the previous year.

Is murder rising in a particular place right now? It’s not an insignificant question. “Violence is often a cascading process,” said Patrick Sharkey, a sociologist at Princeton who built a website to track changes in murder rates for 80 U.S. cities. “When there is a surge of shootings, it can spread and worsen quickly and create a wave of violence. And when this happens in one city, it often is happening in nearby cities as well. Being aware of recent surges, or recent drops, in violence gives local leaders and policymakers a chance to respond.”

So what’s a data analyst to do?

It turns out that there’s a rough workaround: We might need data from only four cities to estimate if murder is up, down or about the same nationally in any given year. The combined murder total from Baltimore, Chicago, Los Angeles and Philadelphia has correctly predicted the national murder direction in every year but one between 1994 and 2017. And these four cities have also presaged what is expected to be a murder drop nationally in 2018 (we’re still awaiting F.B.I. confirmation in two months).