Snow, E. L., L. K. Allen, M. E. Jacovina, S. A. Crossley, C. A. Perret, and D. S. McNamara. “Keys to Detecting Writing Flexibility Over Time: Entropy and Natural Language Processing”. Journal of Learning Analytics, vol. 2, no. 3, Feb. 2016, pp. 40-54, doi:10.18608/jla.2015.23.4.