File:Domestic US airline fare yields, 1955 and 1975.png

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Simat, Helliesen; Eichner, Inc. (January 1976). "Exhibit 6". An Analysis of the Intrastate Air Carrier Regulatory Forum (Report). Vol. 1. hdl:2027/ien.35556029279486.

This work is in the public domain because it was published in the United States between 1929 and 1977 inclusive, without a copyright notice. Unless the author has been dead for several years, it is not in the public domain in countries that do not apply the rule of the shorter term for US works. This includes Canada, China (not Hong Kong, Macao, or Taiwan Area), Germany, Mexico, Switzerland, and other countries with individual treaties. See also further explanation. PD-US Public domain in the United States //en.wikipedia.org/wiki/File:Domestic_US_airline_fare_yields,_1955_and_1975.png

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