Interesting article from a couple years ago:
http://www.pbs.org/newshour/making-sense/how-delta-masters-the-game-of-overbooking-flights/
For the past several years, Delta has largely outperformed its major competitors, United and American, in bumping far fewer passengers involuntarily while routinely getting more passengers to voluntarily fly standby. In 2014, about 96 of every 100,000 Delta fliers had to take a later flight because the plane was overbooked. This compared to 95 at United and 50 at American. But only three of every 100,000 Delta passengers were bumped involuntarily. United had to bump 11 and American, five. Multiplied out, Delta was able to get thousands more of its passengers to agree to stay behind and involuntarily stranded thousands less.
