Justin Reich, a HarvardX Research Fellow, explores the research on video length and impacts on learning and finds that “while this notion of the optimal length of lecture as 5-15 minutes has a lot of traction, it perhaps is more received wisdom than well supported in the literature.” He states that early research focused on attention rather than learning impacts and warns that as new data comes out from platforms like edX, people should “be careful about making the leap between studies of video watching that look at engagement and those that actually look at learning.” He concludes that while MOOCs provide big data sets, it remains observational data and there is a need for more deliberate experimentation to understand how features of course design like lecture length impact not just engagement but also learning outcomes.
http://blogs.edweek.org/edweek/short_videos_are_better_maybe_not.html