 A key innovation in this work was the development of the Classroom Video Analysis (CVA) assessment of teacher knowledge.In the late 2000s Stigler started researching what community college students remember and understand about basic middle- and high-school mathematics, producing several papers (with Karen Givvin) that documented the fragile and fragmented nature of what remains of students’ K12 mathematics education.In the late 2010s Stigler started a collaboration with Ji Son at California State University, Los Angeles.