Zawacki-Richter, Olaf, Eva Maria Baecker, and Anke Hanft. “Validation of Competencies in E-Portfolios: A Qualitative Analysis”. The International Review of Research in Open and Distributed Learning 12, no. 1 (January 31, 2011): 42-60. Accessed June 17, 2019. http://www.irrodl.org/index.php/irrodl/article/view/893.