and sometimes i think policy makers forget that data are not the plural of anecdote the results offer a snapshot of education systems at a certain moment in time but they do not they cannot show how the school systems got to that point or the institutions and organisations that might have helped or hindered progress in addition the data do not really say anything about cause and effect correlations are often deceptive if the birds sing when the sun rises and they do so day after day year after year and in many different places around the world it doesn t mean the sun rises because the birds sing that is one of the main limitations of international surveys and that is where other forms of analysis need to kick in that is also why pisa does not presume to tell countries what they should do