Diriba Gemechu and  Lamessa Abebe,  The Effect of Cooperative Learning on Academic Achievement of Grade Nine Students in Mathematics: The Case of Mettu Secondary and Preparatory School.   journal   = {International Journal of Educational and Pedagogical Sciences}, [online]. World Academy of Science, Engineering and Technology.
    March 2017, vol. 123(3). 725 - 731
    [viewed 26 May 2020]. Available from: https://publications.waset.org/pdf/10006955.