Johnson, Johnson, and Holubec’s (1998) theory has identified three types of cooperative learning groups: formal, informal, and base groups.
Formal cooperative learning groups range in length from one class period to several weeks. The teacher can structure any academic assignment or course requirement for formal cooperative learning. "Formal cooperative learning groups ensure that students are actively involved in the intellectual work of organizing material, explaining it, summarizing it, and integrating it into existing conceptual structures. They are the heart of using cooperative learning" (Johnson, Johnson and Holubec, 1998, p. 1:7).
Informal cooperative learning groups are ad-hoc groups that may last from a few minutes to a whole class period. The teacher uses them during direct teaching (lectures, demonstrations) to focus student attention on the material to be learned, set a mood conducive to learning, help set expectations about material, what the lesson will cover, ensure that students are cognitively processing the material being taught, and provide closure to an instructional session.
Cooperative base groups are "long-term (lasting for at least a year), heterogeneous groups with stable membership whose primary purpose is for members to give each other the support, help, encouragement, and assistance each needs to progress academically. Base groups provide students with long-term, committed relationships." (Johnson, Johnson, & Holubec, 1998, p.1:8).
Johnson, D., Johnson, R.& Holubec, E. (1998).Cooperation in the classroom. Boston: Allyn and Bacon.