According to school principals who participated in PISA 2015, the average student across OECD countries attends a school where 84% of teachers have been fully certified, but this proportion ranges widely, from 100% in Macao (China) to around 10% in Colombia. Findings show that the proportion of teachers who have been certified to teach is positively associated with student performance in only a few selected countries, both before and after accounting for the socio-economic profile of students and schools (Figure 1.9). This overall weak impact is not surprising, given the different standards used in different countries to certify teachers and the fact that most countries have a high percentage of certified teachers.