Reading Mike Mullane's book, Riding Rockets, it seems like shuttle flights were scrubbed, more often than not, during countdown after the crew had boarded, due to some sort of systems failure or weather condition. Some astronauts kept track of their ratios of strap-in to lift-off, with Steve Hawley at one point having been strapped into his seat for launch nine times but only flying twice. Mullane's last flight was in 1990, and I don't know if the track record got better in later years.
Over the course of the Shuttle program, what was the average number of launch attempts per mission?
What mission suffered the greatest number of launch attempts before successfully flying?
Either mean, median, or mode is fine for the average. Getting as far as crew strap-in seems a good definition for a launch attempt, but I'm happy to accept another one.