Umar, B. et al. 2018. DEVELOPMENT OF NATURAL SCIENCE LEARNING INSTRUMENTS WITH CONTEXTUAL APPROACH USING PROBLEM SOLVING MODEL TO IMPROVE CRITICAL THINKING SKILL OF JUNIOR HIGH SCHOOL STUDENTS. JPPS (Jurnal Penelitian Pendidikan Sains). 7, 2 (Jun. 2018), 1501–1506. DOI:https://doi.org/10.26740/jpps.v7n2.p1501-1506.