Sample-based prospective longitudinal survey Survey design: Mixed mode (CATI interview & written questionnaire) Population (1st cohort TREE1): Youth who have completed compulsory school in 2000. Sample design: stratified sample; Sample size: valid sample at outset (2001): 6.343; Survey rhythm: base survey (PISA) in 2000, 9 follow-up survey waves 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2010 and 2014; Field work: Polling institute