Spirit Airlines is guilty of bumping more passengers than any other U.S. airline, a new report said. Around 15,000 people out of 630 million travelers have the unfortunate experience of being bumped from a flight due to overbooking, but passengers on Spirit Airlines experience it the most.

Data from the U.S. Department of Transportation found, April 2017–April 2018, that Spirit Airlines involuntarily bumped passengers at a rate of 78 per million boarded, followed by Frontier Airlines (55 per million), Southwest Airlines (41 per million), Alaska Airlines (28 per million) and American Airlines (24 per million).

Airlines are legally allowed to sell more tickets than they have seats on planes, but most offer compensation to travelers who get bumped due to overbooking. Airlines offer incentives. like cash and mileage, to get passengers to voluntarily give up seats before they move to involuntary removal, always a last resort.

Southwest Airlines bumped the most passengers in terms of sheer numbers, at 6,411. But it also boarded the most people, at 157.7 million.

The lowest rates came from American Airlines, which bumps only three travelers per million boarded, and JetBlue, which bumps two passengers per million boarded.