April 8 (UPI) -- U.S. airlines have posted their best service ratings in nearly 30 years of study, according to a yearly review of all nine major carriers.
The latest Airline Quality Rating on Monday showed ratings have improved for five straight years. The survey concentrates on four areas -- on-time arrivals, mishandled bags, bumping passengers and complaints filed. This year's report shows U.S. carriers posted their best service marks in the study's 29-year history.
"The Airline Quality Rating industry score for 2018 shows an industry that improved in overall performance quality over the previous year," the survey said.
Six airlines -- Delta, Hawaiian, JetBlue, Southwest, Spirit and United -- showed improvement in their scores over last year. Low-cost carrier Spirit showed the greatest improvement. Three airlines -- Alaska, American and Frontier -- saw their scores decline.
The report emphasized the two largest improvements -- complaints to the Transportation Department fell by 23 percent and the number of passengers bumped from oversold flights was cut in half.
Delta earned the highest score -- up from second place last year -- and was followed by JetBlue and Southwest. Alaska Airlines was rated fourth, followed by Hawaiian, United, Spirit, American and Frontier.
The annual report is conducted by professors Dean Headley at Wichita State University and Brent Bowen of Embry-Riddle Aeronautical University.
"It is the best year we ever had," Headley told CNBC. "Three out of the four things that we track got better this year."
What the study does not account for, they said, are complaints submitted via social media. Complaints online have become more of an effective way for passengers to speak up, because they often receive immediate attention from airline personnel.