The Leaving Certificate, which is taken after two years of study, is the final examination in the secondary school system. Those intending to pursue higher education normally take this examination, with access to third-level courses generally depending on results obtained from the best six subjects taken, on a competitive basis. Third-level education awards are conferred by at least 38 Higher Education Institutions - this includes the constituent or linked colleges of seven universities, plus other designated institutions of the Higher Education and Training Awards Council. The Programme for International Student Assessment, coordinated by the OECD, currently ranks Ireland as having the fourth highest reading score, ninth highest science score and thirteenth highest mathematics score, among OECD countries, in its 2012 assessment. In 2012, Irish students aged 15 years had the second highest levels of reading literacy in the EU. Ireland also has 0.747 of the World's top 500 Universities per capita, which ranks the country in 8th place in the world. Primary, secondary and higher  level education are all free in Ireland for all EU citizens. There are charges to cover student services and examinations. In addition, 37 percent of Ireland's population has a university or college degree, which is among the highest percentages in the world.. How many ranks higher does Ireland rate among other OECD countries in mathematics compared to science?