• - Geometry Data: Natural Earth Data, Countries in 1:10’000’000, https://www.naturalearthdata.com/downloads/10m-cultural-vectors
• - GDP: United Nations. http://data.un.org/_Docs/SYB/CSV/SYB63_230_202009_GDP%20and%20GDP%20Per%20Capita.csv
• - Tertiary Education: United Nations. http://data.un.org/_Docs/SYB/CSV/SYB63_309_202009_Education.csv
• - Expenditures on Education: United Nations, http://data.un.org/_Docs/SYB/CSV/SYB63_245_202009_Public%20Expenditure%20on%20Education.csv
• - Pisa Data: OECD, https://www.oecd.org/pisa/data

• Boutayeba, F. and Ramli, M. (2019). The Link Between Education and Economic Growth in Algeria: An Empirical Investigation. International Journal of Advanced Research in Education and Society 1(1), pp. 35–43. (Available online at myjms.mohe.gov.my).
• Chatterji, M. (1998). Tertiary education and economic growth. Regional Studies, 32(4), pp. 349–354. doi: 10.1080/00343409850117807.
• Bloom, D., Canning, D. and Chan, K. (2006). The Modern Shift From Luxury Goods to Luxury Experiences. The World Bank. (Available online at edu-links.org).
• Lupu, D. et al. (2018). The Impact of Public Expenditures on Economic Growth: A Case Study of Central and Eastern European Countries. Emerging Markets Finance and Trade 54(3), pp. 552–570. doi: 10.1080/1540496X.2017.1419127.

The webpage design is based on the free CSS template "Perfect Learn" by free-css.com and is provided under the Creative Commons license; this includes the respective images.
The three images of early maps on the contact page are all in the public domain:

• - Peutinger map (original from around the 4th century A.D., copy from 13th century)
• - Tabula Rogeriania, from the Muslim scholar al-Idrisi, drawn in the 12th century A.D. as part of the geography book "A Guide to Pleasant Journeys into Faraway Lands" compiled for the Norman King Roger II
• - Map produced in 1569 by the Flemish cartographer Gerardus Mercator using a new type of projection which today bears his name

(all of the above found on history.com.