Author's accepted version (post-print).This is the peer reviewed version of the following article: Grepperud, S., Mathisen, T. A., Pedersen, P. A. (2019). Accreditation in a differentiated duopoly. Managerial and Decision Economics, which has been published in final form at https://doi.org/10.1002/mde.3005. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions.Available from 13/02/2021.