This study tested a model of teacher interventions (TIs) conducted during cooperative learning to examine how they affected students' subsequent time on-task (TOT) and problem solving. TIs involved groups of ninthS uppose you are a teacher and have several groups of students working on an algebra problem. Some groups are working productively, while others are not. When do you intervene? How should you intervene? What kinds of interventions are likely to improve students' problem solving?In the present study, I addressed these questions by building on past research on cooperative learning (CL). A group of students engaged in CL works together to achieve shared goals. Ideally, students help one another learn MING MING CHIU is an associate professor at the Chinese University of Hong Kong, Department of Educational Psychology, 314 Ho Tim Building, Shatin, NT, Hong Kong; e-mail mingming@cuhk.edu.hk. Using advanced statistical methods, he examines group processes during cooperative learning and compares student learning in large-scale international studies.