such improvement could indicate that countries are harvesting some of the early fruits of their efforts to improve their education systems but sustained improvement over several years and pisa assessments is much more difficult to achieve colombia and portugal are among the few education systems whose reforms have been successful in improving average student performance in science over successive pisa cycles in beijingshanghaijiangsuguangdong china hereafter bsjg china and viet nam for instance there are fewer lowachieving students in school than on average across oecd countries but the pisa target population represents less than of the overall population of yearolds in viet nam and only in bsjg china