This report examines the performance of students enrolled in the IB Primary Years Programme (PYP) and the IB Middle Years Programme (MYP) on the ACER International Schools’ Assessment (ISA) compared with non-IB students from the same ISA cohorts. The ISA is an assessment created especially for students in international schools in Grades 3 to 10. The assessment asks both multiple-choice and open-ended questions in the areas of writing, reading, mathematics and science, and provides international normative information about student performance. The ISA scales for Reading, Mathematical Literacy and Scientific Literacy are based on those developed for the internationally endorsed frameworks of the OECD's Programme for International Student Assessment (PISA). The IB – non-IB comparison studies, based on students who participated in the ISA were completed in 2009 and in 2011. The current study is a follow-up which was based on students who participated in ISA assessments in 2017-18 and 2018-19. This study replicates previous studies using ISA October 2017/February 2018/May 2018 and October 2018/February 2019/May 2019 data.