From the
bitre page on domestic airfares
Notes
Indexes are constructed from BITRE's monthly survey of airline internet booking sites. Fares are recorded only when they are available on the nominated day of travel (the last Thursday of the month). The series is a price index of the lowest available fare in each fare class, weighted over selected routes. It does not measure real airline yields, or average fares paid by passengers. For more information on methodology use the link at the top of this page.