About 15,000 travellers out of 630 million are unlucky enough to be involuntarily bumped off of their flights due to airline booking, and Spirit Airlines is the most likely carrier to bump passengers, Money reports.

Data from the US Department of Transportation found that in the year ending April 2018, 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 on flights than seats available, but must compensate travellers who are bumped from flights due to overbooking.

Normally, airlines offers a variety of cash and other incentives to get travellers to voluntarily give up their seats on flights. Involuntary bumping can occur when volunteers cannot be found.

In terms of sheer numbers, Southwest Airlines bumped the most passengers – 6,411. But it also boarded the largest number of passengers, 157.7 million.

Delta Air Lines, which boards passengers at a rate comparable to American Airlines, only bumps about three travellers per million boarded. And Jetblue bumps just 2 passengers per million.