Wouldn't pay too much attention to what's basically a survey,
From the outset, Pisa has been met with scepticism, criticism and even outrage, most of which has stemmed from the claim that the study's findings are arbitrary. One such voice was Dr Svein Sjøberg of the University of Oslo, who claimed that a small change in question choice or weightings could result in a big change in a country's overall rankings.
Though the methodology for collecting the results might be clear, the way they are interpreted and analysed to become final results is less so. This lack of statistical transparency has also been a focal point of criticism levelled at Pisa and indeed the OECD
The sheer breadth of Pisa has also raised questions about the comparability of results within it.
At a more fundamental level, some have pointed to the difficulties of testing students in such a wide array of languages. If such a thing as a perfect translation exists (few would argue it does) it might still not be enough to control for students' differing interpretations of the same set of instructions.
There is also a temptation to grab at averages from Pisa to understand the performance of a country. Doing so would obscure the vast differences that can occur regionally within a country - such an oversight can be particularly dangerous, say critics, for understanding the true academic opportunities available in a country.
Plus PISA is just a wing of the OECD anyway and the question should be what agenda is the OECD trying to push by publishing PISA reports?