We propose to assess the influence of a number of events on the degree of competition in the Dutch electricity wholesale market over the period 2006-2011 through a decomposition method based on the residual supply index. We distinguish regulatory market-integration events, firm-level events and changes in the level of residual demand. We conclude that market-integration measures to improve competition have been effective, but that changes in residual demand appear to have been equally important. Firm-level events have only had a minor impact on the intensity of competition. (c) 2013 Elsevier B.V. All rights reserved.