I think you have to go with revenue passenger miles (i.e. paying passengers X miles flown). It gives the product of passengers and miles that Merry was talking about. Delta flew 103,342 million (103 billion) revenue passenger miles in 1998. United flew 124 billion rpm in 1997 and it looks like they will surpass that this year so UA is the largest by that measure. It means that Delta is flying shorter flights with more people while UA is flying longer flights with fewer people.

very interesting Roti--I had some odd things happen in that timeframe with my MP account myself. In '92-'93 I was based in Chicago, but could use any airline, so used DL and AA.
(had some grudges against UA after the '85 strike...but that's another story) In '94, based in Des Moines, was Medallion with DL. In '95, returned to CHI, same company which had done a deal with UA. (Didn't argue--we got the discount on personal travel). Mysteriously in mid'95, got a Premier card. Was downgraded in '96 to Premier Emeritus, gained it back late '96, Premier '97 and most of '98.

I attributed it to our company's choice of UA for the contract??? Maybe I was wrong!