The general statistic that has been around for a while says that 10% of pax booked on a flight will not show up. The airlines track no-show factor for each flight, market, date/time, holiday period, etc. They have sophisticated computer programs, and departments full of analysts, who do nothing but try to predict how many seats they can sell on the flight so it will go out full, and to maximize the highest possible fares on said flights. I too am curious if electronic ticketing has reduced the no-show factor on flights in general.

Generally if a flight over-sells by 2 to 4 passengers, the airline is still making out ahead on the flight, especially if they pay-off the pax voluntarily with vouchers and the like.

Carriers will also adjust booking levels in response to weather, e.g. they will program computers to not over-sell flights anymore the day after a storm...because they know the no-show factor is going to be less in that circumstance. They weight a number of things on a flight to figure out the appropriate no-show factor. How did the flight do yesterday, a week ago, a month ago, last year on todays date, factor in holidays, etc.