Have there been 353 mass shootings this year — or just 4?
The shooting in San Bernardino, California, on Wednesday was the 353rd mass shooting of 2015, according to the crowdsourced Mass Shooting Tracker that Vox uses for our maps documenting mass shootings. Or it was the 29th, if you use data from USA Today. Or it was the fourth, if you use a database maintained by Mother Jones.
How are three news outlets coming up with such different answers? It all comes down to definitions: