1993-2004 (Tallent-Runnels [10] )

2000-2008 (Zawacki [12] )

2009-2018 (Martin [11] )

Most number of studies

•Course environment

•Learner outcomes

•Interaction and communities of learning

•Instructional design

•Learner characteristics

•Engagement

•Learner characteristics

•Evaluation and quality assurance

Lowest number of studies

•Learner characteristics

•Institutional and administrative factors

•Management and organization

•Research methods in DE and knowledge transfer

•Globalization of education and cross-cultural aspects

•Course design and development

•Leadership, policy, and management

•Instructor characteristics