Thank you for your comment, and careful reading. You are correct that I did not rescale the scores. I took them as raw. I wanted to see who improved among those that participated in both 2012 and 2000 and did the same simple calculation for all three subjects, despite the warnings from the OECD. Given the many comparability issues involved in international assessments -- see for example http://www.economist.com/blogs/economist-explains/2013/12/economist-explains-4?fsrc=scn/tw_ec/how_accurate_are_school_league_tables _-- I am not sure my transgression is all that serious. In any case, the increase in scores for Peru even if one only takes one subject are nonetheless impressive. In the coming months, and years, we along with many others, will take a careful look at PISA and try to explain improvements.
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