e;learning is very hard to circumscribe. Do you vant to speak about the new tech evolution (so you can speak on e.learning in the 60' to our days), Do you want to speak about the e.learning market ? about the political point of view (the place of e.learning for the nation evolution...) ? e.learning for student ? the student activity when they are using e.learning ? the e.learning performance (and the hudge problem of the evaluation). about the teacher's activity ? The influence of digital interface on learning ? The collaborative activity between learners......
The answers depend of the point of view you choose.
I think you d better be more precise in your question. Like the above speaker I think you have to narrow it down substantially. Most reviews deal with the question on effeciency and effectiveness. Margeret Cox, Kings College in London (UK) and Jeff Moonen (emiritus) , University of Twenthe (NL) dealt in depth on these issues.
See: Clark, R. E., Yates, K., Early, S. & Moulton, K. (2010). An analysis of the failure of electronic media and discovery-based learning: Evidence for the performance benefits of guided training methods. In K. H. Silber, & R. Foshay, (Eds.). Handbook of training and improving workplace performance, Volume I: Instructional design and training delivery. Washington, DC: International Society for Performance Improvement.
I would suggest to check the two volumes book which was published on e-learning across multi countries. I published one of these chapters. You can download both volumes from the link
I agree that the question needs to be more specific as there are many different types of e-learning. Also, is e-learning an effective tool for all subjects. There is some research, but not much, as online learning is still a somewhat new platform. We know from Coursera, etc, that many more people register than complete. At most colleges a minimum 2.5 GPA is required because of the internal discipline and motivation necessary to complete an online course. Anecdotal feedback from young people to me has been that they prefer being in the classroom and put less effort into online learning Hardly empirical data, but I think important info to think about, especially as our students have become guinea pigs in a giant experiment pushed largely by for profit tech companie; in my opinion always a bad idea for education.