https://www.eurofound.europa.eu/sites/default/files/ef_publication/field_ef_document/ef1708en.pdf
https://www.ipg-journal.de/regionen/global/artikel/detail/vorbei-mit-der-freiwilligkeit-4508/
https://www.esri.ie/system/files/publications/JR2.pdf
https://www.boeckler.de/pdf/p_study_hbs_376.pdf