A viral video of a man being dragged off a United Airlines flight sparked outrage and fueled widespread criticism of airlines forcing passengers to surrender seats.

In fact, the numbers of people who are involuntarily bumped off a flight tell a different story.

Those numbers, already minuscule compared to the millions of people who fly domestically each day, have dropped over the past four years in part because airlines are honing their ability to predict which flights to overbook to make up for no-show passengers. Delta Air Lines has one of the lowest rates for involuntarily bumping passengers, while United Airlines, which is now reeling from a public-relations fiasco, isn't far behind.

Four years ago, Delta Air Lines involuntarily bumped 6 passengers per 100,000, a number that they brought to just 1 per 100,000 last year, according to national statistics. In that same time period, United Airlines dropped its numbers from nearly 12 per 100,000 to just four. Both airlines have rates below the industry average.

"Of course that doesn't matter," said Zach Honig, editor in chief of travel website thepointsguy.com. "It's hard to look away [from the United video]. … Usually people aren't dragged off a plane unless they're drunk or they've assaulted another passenger."

Until that video blew up the internet and sparked international outrage, few passengers likely thought they would be told to involuntarily give up their seat to make room for someone else.

Over the past four years, airlines — including United Airlines — have done a better job of avoiding bumping their passengers.

"People don't know that it's been going on for years," said Dean Headley, co-founder and co-author of the Airline Quality Rating. "It just doesn't come up that much. But it happens more than people might see because it's usually done quietly and most often before boarding."

It's all part of the gambit that allows airlines to make a profit while trying to keep fares down.

To make a profit, airlines have to fly 80 percent full, Headley said. To keep those numbers up, airlines overbook some flights by using sophisticated data to predict no-show passengers who bought premium refundable tickets.

But if the gamble doesn't work, airlines bump passengers. Airlines also bump passengers if they unexpectedly have to fly a smaller plane than originally planned. In a case like the United flight that made headlines earlier this week, airlines may have to make room for employees needed to crew a plane at another airport.

Sometimes, there's little wiggle room because airplanes are more full than they have ever been. Last year, more than 83 percent of the seats on U.S. airlines were occupied.

To make room, airlines most often ask for volunteers to give up their seats, putting them on another flight and tossing in added compensation such as vouchers for future flights.

"They almost always get volunteers," Honig said. On a recent flight from Hawaii, Honig took a $1,000 voucher for his seat and grabbed a flight home an hour later.

It's a customer-relations plus to get volunteers rather than force someone to give up their seat, Headley said.

Overall, Delta bumped far more passengers than other domestic airlines, but it did a much better job at getting passengers to voluntarily give up their seats. As a result, the carrier's involuntary bumps rank among the best.

When there are no volunteers, the airlines will choose passengers to involuntarily give up their seats. The compensation depends on various factors, and goes up to a maximum of $1,350, according to the Fly Rights published by the U.S. Department of Transportation.

Passengers have to remember that airlines basically reserve the right to refuse service to anyone, Headley said.

Even so, very few airline customers are put in that situation — maybe about 200 a day of about 3.5 million people who fly daily in the U.S., Headley said.

The unlucky passengers are usually the ones who bought the cheapest tickets and were among the last to check in, whether it be online or in person at the airport, Headley said.

"It just depends where you are on the food chain," he said.

Staff writer MaryJo Webster contributed to this report.

Mary Lynn Smith • 612-673-4788