Kilde: <a href="https://no.wikipedia.org/wiki/PISA-studien_2009"><strong>PISA</strong></a><strong>, <a href="http://www.oecd.org/pisa/keyfindings/pisa-2012-results-overview.pdf">PISA</a>, <a href="https://www.nrk.no/norge/pisa-2013_-darligere-i-realfag-1.11393522">NRK</a></strong>