FiveThirtyEight examines Has Expanded Replay Worked Well In Baseball?. "As of Sept. 7, there had been 1,130 challenges. Of these, 529 (or 47 percent) resulted in a call being overturned. Managers initiated 83 percent of the challenges, umpires 17 percent."
I'm most surprised that 19 Hit by pitch calls have been overturned. How does that work?