New evidence presented in this paper shows that existing accounts of Programme for International Student Assessment (PISA) in developing countries underestimate both progress and inequality because they do not take into account the large proportion of 15-16-year olds who are ineligible for the PISA sampling frame, primarily due to dropout or delay. In such countries, who makes it into PISA (sample eligibility) heavily influences representivity and therefore the interpretation of PISA results. We use PISA in Turkey 2003-2012 as a case study to show how combining measures of access (from the Turkish Demographic and Health Surveys) and learning outcomes (from PISA) can account for changes in the underlying population. We find the percentage of 15-16-year olds reaching Level 2 in PISA is up to twice as large as that reflected in official PISA reports. These findings have strong implications for inter-country and inter-temporal comparisons using PISA, particularly for developing countries.