http://www.iata.org/publications/economic-briefings/value-of-air-cargo-2016-summary.pdf
https://www.veblen-institute.org/IMG/pdf/aubert_hdef.pdf
https://www.nber.org/system/files/working_papers/w30028/w30028.pdf
https://www.fraserinstitute.org/sites/default/files/economic-freedom-of-the-world-2021.pdf