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 Beijing-Shanghai-Jiangsu-Guangdong (China) (hereafter "B-S-J-G [China]") and Viet Nam, for instance, there are fewer low-achieving students in school than on average across OECD countries, but the PISA target population represents less than 30% of the overall population of 15-year-olds in Viet Nam, and only 64% in B-S-J-G (China).