These are the nation's largest U.S. airlines, ranked from worst to best by new study

Julia Thompson

Delta Air Lines has topped yet another airline ranking, this time the 29th annual Airline Quality Rating, which was released Monday. 

The rating examines the nation’s largest nine airlines based on data collected in 2018 of mishandled baggage, consumer complaints, on-time performance and involuntary denied boardings.

The AQR claims to be the most comprehensive study of performance and quality of the U.S.'s largest airlines and is a joint research project of the W. Frank Barton School of Business at Wichita State University and Embry-Riddle Aeronautical University’s Prescott, Arizona, campus. 

Delta was No. 2 in last year's AQR ranking, behind Alaska Airlines. But this year, Alaska fell to No. 4. The study notes that since the merger of Alaska and Virgin America was completed in April 2018, results for Alaska in the 2019 ranking reflect the performance of the merged airlines for the entire year.

In other 2019 airline rankings, Delta was also rated the top airline in the U.S. in The Point's Guy's third annual list; Frontier was rated the worst on both lists. TripAdvisor's Travelers’ Choice awards for Airlines, which ranks airlines worldwide, had Singapore Airlines on top. Southwest Airlines was the only U.S.-based airline to make that list, coming in at No. 6. 

The Points Guy: These are the best — and worst — airlines in the U.S.

TripAdvisor awards: These are the 10 best airlines in the world

The AQR study does more than just rank airlines; it also tracks overall trends within the airline industry. The study notes that three of the four AQR data points tracked – involuntary denied boardings, mishandled bags and customer complaints – improved for the industry as a whole in 2018. But on-time performance, the most heavily weighted element in the study, slipped.

"These results are very needed by the traveling public, given all the recent reports of airlines losing the confidence of key consumers," said Brent Bowen in a news release. Bowen is the AQR co-author and a professor of aeronautical science at Embry-Riddle’s Prescott campus. "Travelers can be confident that our airlines are improving."

This year’s AQR report also showed the lowest rate of bumped passengers, the lowest rate of mishandled baggage and the lowest rate of customer complaints for the industry since the study began in 1991.

Keep in mind this is 2018 data, which doesn't reflect issues airlines and passengers have encountered this year related to the March 13 grounding of the Boeing 737 Max 8

Scroll through to see the full AQR ranking from worst to best below. 

More flight-related rankings

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TripAdvisor awards: These are the 10 best airlines in the world

OAG data: These are the world's top 20 international airline routes

OAG data:The 20 most popular domestic flights around the world

Skytrax awards:These are the world's top 25 airports for 2019

Fundera:Best — and worst — U.S. airports for business travelers

FlyerTalk Awards:The top airline, hotel and rental car rewards programs

This article originally appeared on USA TODAY: These are the nation's largest U.S. airlines, ranked from worst to best by new study