http://www.niesr.ac.uk/sites/default/files/publications/Facing%20the%20Future%20Tackling%20post-Brexit%20Labour%20and%20skills%20NIESR.CIPD_.pdf
http://www.ilo.org/wcmsp5/groups/public/—dgreports/—dcomm/—publ/documents/publication/wcms_548950.pdf
https://www.ipg-journal.de/regionen/global/artikel/detail/vorbei-mit-der-freiwilligkeit-4508/