Griffiths, Alan; Wall, Stuart (2007). Applied Economics (11th izd.). Harlow: Financial Times Press. str. 6. ISBN978-0-273-70822-3. Arhivirano iz originala na datum 2009-08-23. Pristupljeno 26 December 2010.
Griffiths, Alan; Wall, Stuart (2007). Applied Economics (11th izd.). Harlow: Financial Times Press. str. 6. ISBN978-0-273-70822-3. Arhivirano iz originala na datum 2009-08-23. Pristupljeno 26 December 2010.