Can we quantify the difference made by or popularity of online learning? How much of a change is online learning through open courseware (OCW), etc., actually making? Are there any peer reviewed studies regarding the same?
One way is to assess the performance by a single test of two groups of students, one in the regular classroom atmosphere and the other group using solely online tools. The topic may be chosen at random, with varying levels of difficulty. It would also provide information about effectiveness of using online tools in teaching difficult concepts. This study by Anna Ya Ni  is in that direction.