When compared to more traditional methods where students non-interactively receive information from a teacher, cooperative, problem-based learning demonstrated improvement of student engagement and retention of classroom material. Additionally, academic achievement and student retention within classrooms are increased. A meta-analysis comparing small-group work to individual work in K-12 and college classrooms also found that students working in small groups achieved significantly more than students working individually, and optimal groups for learning tended to be three- to four-member teams with lower-ability students working best in mixed groups and medium-ability students doing best in homogeneous groups. For higher-ability students, group ability levels made no difference.