^a For air carriers, the data were dampened, or smoothed, to reduce the month-to-month fluctuations. This dampening was performed using an exponential smoothing model, with a weight of 0.95. Departure data, and hence the denominator of the rates, are not strictly comparable between pre- and post-1977 eras.

Source: Various sources, as cited in U.S. Department of Transportation, Bureau of Transportation Statistics, National Transportation Statistics 2001, available Spring 2002 at www.bts.gov, or email to answers@bts.gov.