The statistics are grim. There is a roughly 60% chance that today there will be at least one mass shooting in the US. And there is a 17% probability that there will be exactly two shootings.

This weekend, at least 29 people were killed in two mass shootings within 24 hours in the US. The back-to-back events, in El Paso, Texas and Dayton, Ohio, prompted widespread shock, but according to data collected by the Gun Violence Archive, the phenomenon isn’t new.

Since 2014, there have been 334 mass shootings per year, on average, in the US. (The Gun Violence Archive defines a mass shooting event as one in which at least four people were killed or injured.) With so many mass shootings every year, the probability is high that on any given day there is more than one incident.

Since 2014, there have been 484 individual days with two or more mass shootings—on 407 of those days, at least one person was killed. And that doesn’t include events like this past weekend, when shootings happen within 24 hours but on consecutive days.

The deadliest mass shooting in American history—the 2017 Las Vegas massacre that killed 59 people—wasn’t the only incident that day. Three people were killed and two injured in a shooting in Lawrence, Kansas on the same day.

Mass shootings have become so commonplace in the US that even the hourly probabilities are shocking. All else being equal, during the typical eight-hour work day, there is a one-in-four chance that at least one mass shooting incident will occur, and a 3% probability that there will be two.