But American politicians fighting over the issue often fail to effectively or accurately talk about that data. Concentrated bursts of violence take over the narrative and are exploited for political points. Weekends like the one Chicago just saw are heartbreaking, and they speak to the deep social problems here.

Yet they aren’t, in and of themselves, a trend any more than a particularly destructive winter storm represents the overriding trend in rising global temperatures. Crime is more like weather, influenced by the ebb and flow of individual, isolated phenomena but not defined by them and rarely directly controlled by them.

That doesn’t stop politicians of different stripes from either declaring data trends that in fact don’t exist or finding easy answers to explain those that do.

In 2015, Mayor Emanuel, a Democrat, blamed police officers’ becoming “fetal” for an uptick in violent crime, stating that “what happened post-Baltimore, what happened post-Ferguson is having an impact.” However, while Chicago’s violent crime was about 2.5 percent higher in 2015 than in 2014, it was still lower than in any year from 2001 to 2013.

This week, the Democratic candidate for governor, J.B. Pritzker, blamed Gov. Bruce Rauner’s budget cuts for the violence: “It’s true that our (increased) violence around the state of Illinois, not just the city of Chicago, has been almost concurrent with the defunding of those services that people rely upon.”

The cuts may be troubling to local progressives, and crime at any level is regrettable. Still, Mr. Pritzker’s statement is a broad brush considering violent crime in Illinois is down this decade compared with both the 2000s and the 1990s — a period of interchanging party rule.