Here's how airlines like United choose who to kick off of a flight

On Sunday, law enforcement officials forcibly removed a passenger from a United Airlines flight in Chicago.

The incident stemmed from the passenger's refusal to leave the plane after being bumped from Flight 3411 to Louisville, and it highlights the practice bumping ticketed passengers from flights.

In many instances, airlines oversell their flights, using algorithms that calculate the likelihood people will cancel, not show up, or run late. However, there are times where passengers call the airlines' bluff and they all show up for the flight.

That's actually not what happened here. A United Airlines spokesperson told Business Insider that Flight 3411 was not overbooked — in contradiction to a statement released by the airline Monday morning. Instead, the airline needed to bump four passengers from to flight in order to make room for pilots and crew it needed to transport down to Louisville to operate flights later on that evening.

See how people reacted to the incident on Twitter:

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People react to United Airlines kicking a passenger off a flight
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People react to United Airlines kicking a passenger off a flight
Pepsi: We are the most hated company right now. 😓 United Airlines: 1 sec - Hold my Pepsi. #PEPSI #unitedAIRLINES
Not so friendly skies huh @united ? Makes me sick now how you treat customers. I hope he sues hard. #unitedairlines https://t.co/D13SDvHfLq
You want passengers to give up their seats because you have piss poor planning & then beat them up? Shame on you. @united #unitedAIRLINES
Apparently @united 's inability to coordinate a flight makes violent assault against customers ok. Never flying #UnitedAirlines again
NEVER flying @united ever again! Horrific, brutal, intolerable treatment of passenger, + tone-deaf "apology" from CEO. #UnitedAirlines
Dear United Airlines: People have smart phones.
Perhaps @MerriamWebster can help #United Airlines with the definition of "volunteer"
United Airlines to United Airlines: "I see your legging-gate, and I raise you a passenger beating" #UnitedAirlines #United #flight3411
Look on the bright side, United Airlines. After this incident, you'll never have to worry about a flight being overbooked again.
You thought United Airlines was tough on folks who wear leggings. Just wait to see what they do to folks in sweat p… https://t.co/x5Nzp7gcn1
Poor planning on your part doesn't constitute an emergency on ours. Fly Southwest. Fly Delta. Hell, fly Spirit. Not #UnitedAirlines
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"What happened with United was exceedingly rare," aviation analyst Henry Harteveldt told Business Insider.

Regardless of the cause, though, airlines follow a specific procedure when they've got more passengers than they can carry.

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First, airlines will ask for volunteers,— through email, at check in or even at the gate. These requests for volunteers will typically come with anything ranging from large sums of cash to hotel rooms and a first class upgrade on a later flight.

According to Harteveldt, there is no federal limit to amount of money an airline can offer its passenger to deplane.

But this doesn't always solve the problem.

When there are simply not enough volunteers, an airline can deny boarding to any remaining passengers.

Statistically speaking, this rarely happens. According to Wichita State University and Embry Riddle University's Airline Quality Rating study, involuntary denied boardings fell to just 0.62 per 10,000 passengers last year.

Unfortunately for the Louisville-bound passenger, he was selected as one of the passengers to be bumped.

His selection, though, wasn't random. He was chosen based on a series of criteria.

Based on United Airlines' Contract of Carriage (more on that later), passengers with disabilities and unaccompanied minors are the least likely to bumped from a flight.

United Contract of CarriageUnited Airlines

For everyone else, the contract states that the airline's decision is based on a passenger's frequent flyer status, the layout of his or her itinerary (whether the passenger has a connecting flight), the fare class of the ticket, and the time he or she checked into the flight.

This means passengers who bought more expensive tickets, have higher frequent flyer status, or checked in early are less likely to be bumped.

Delta and American both operate under similar policies as stipulated in their respective contracts of carriage.

A Contract of Carriage

Buying a plane ticket isn't so much a straight-forward purchase as it's an agreement to adhere to the airline's contract that spells out the terms of service for the flights. The agreement is called the Contract of Carriage.

"Customers do agree to a contract of carriage when they purchase a ticket with clear stipulation and it's available on United.com," an airline spokesperson told Business Insider.

Every airline has one and they are usually available on their website. They are long, complex, and filled with legalese. For instance, Delta's contract is 51 pages long.

As with any contract written by a single party in that agreement, it's heavily tilted towards the protection of the airlines' interests.

Do you airlines have the right to throw you off of their planes even if you haven't done anything wrong?

"Yes," Harteveldt said. "Remember, it is their aircraft and their seat — you're just renting it to get from point a to point b."

NOW WATCH: People are outraged by this shocking video showing a passenger forcibly dragged off a United Airlines plane

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SEE ALSO: United CEO's response to passenger being dragged from plane made the situation even worse

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