https://www.nao.org.uk/wp-content/uploads/2018/01/PFI-and-PF2.pdf
https://www.dossier.at/dossiers/aktuelles/zu-kuehl-um-wahr-zu-sein/
https://www.fraserinstitute.org/sites/default/files/economic-freedom-of-the-world-2021.pdf
https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3305056