https://www.ipg-journal.de/regionen/global/artikel/detail/vorbei-mit-der-freiwilligkeit-4508/
http://www.iata.org/whatwedo/Documents/economics/impact_of_brexit.pdf
http://researchbriefings.files.parliament.uk/documents/CBP-7484/CBP-7484.pdf
https://www.boeckler.de/pdf/p_wsi_report_39e_2018.pdf