Animated flashcards and sub-two-minute videos turn out to be the most effective online resources for K-12 against all other common options. That's the finding determined by OpenEd, which recently studied the data generated from the results of assessments given to students who used its free online resources for educational purposes. The company examined the average "effectiveness" of its online instructional resources. That was defined by how well students performed on an assessment covering the same subject or standard category after viewing the resource.
Schaffhauser, D. (2015, September 23). OpenEd Assesses 'Most Effective' Online Learning Resources. The Journal. Retrieved from https://thejournal.com/articles/2015/09/23/opened-assesses-most-effective-online-learning-resources.aspx
Paper cited can be accessed at https://dl.dropboxusercontent.com/u/9002954/ResourceEffectiveness.pdf