Seven airlines improved mishandled baggage rates in 2011: Alaska, American, Atlantic Southeast, Delta, Frontier Airlines, JetBlue and SkyWest Airlines. The mishandled baggage rate for the industry decreased from 3.49 per 1,000 passengers in 2010 to 3.35 last year.Southwest Airlines once again had the lowest consumer complaint rate, 0.32 complaints per 100,000 passengers; United Airlines had the highest consumer complaint rate at 2.21.Headley attributed United's high complaint rate to rough patches in the airline's merger with Continental. The airlines, which now operate under the United name, merged reservation operations last month.The overall rate of customer complaints was virtually unchanged, 1.19 in 2011 compared to 1.22 in 2010. Only five of 15 airlines improved their customer complaint rates for 2011 - AirTran, Delta, Frontier, Hawaiian and JetBlue.The majority of complaints to fell into three categories: flight problems, 34.9 percent; baggage, 14.3 percent; customer service, 12.1 percent, and reservations, ticketing and boarding, 11.2 percent.The research was sponsored by Purdue, in Indiana, and Wichita State.