http://www.case-research.eu/files/?id_plik=5223
http://www.ebrd.com/news/events/ebrd-transition-report-201718.html
https://www.dgb-bestellservice.de/besys_dgb/pdf/DGB15130.pdf
https://www.smmt.co.uk/wp-content/uploads/sites/2/SMMT-Sustainability-Report-2018.pdf