These indexes are calculated using ASQP data reported to the Bureau of Transportation Statistics on both scheduled and actual flight times (based on gate-departure and gate-arrival) for all domestic nonstop flight segments flown by U.S. carriers with at least 1 percent of passenger revenues in the previous year. Actual (or scheduled) travel time is the difference in minutes between the scheduled departure and the actual (or scheduled) arrival. Air Travel Time Variability is measured using standardized deviations of actual air travel time from its average value. Deviations are weighted to increase the variability index more for extreme deviations than for small deviations. All of the Air Travel Time Indexes are designed to control for changes in carriers, routes, and time of departure in order to improve the reliability of comparisons over time. Airports included in the analysis are those that ranked in the top 50 (by passenger enplanements of all large certified carriers) for at least one year between 1990 and 2002. The time-of-departure periods are: morning offpeak (before 9 a.m.); mid-day peak (between 9 a.m. and 3 p.m.); evening peak (between 3 p.m. and 9 p.m.); and evening offpeak period (after 9 p.m.).