Southwest maintains excellent customer satisfaction ratings; for many years, according to the Department of Transportation Southwest ranked number one (lowest number of complaints) of all U.S. airlines for customer complaints. Southwest Airlines has consistently received the fewest ratio of complaints per passengers boarded of all major U.S. carriers that have been reporting statistics to the Department of Transportation (DOT) since September 1987, which is when the DOT began tracking Customer Satisfaction statistics and publishing its Air Travel Consumer Report.