The purpose of this article is to examine phase-differences in the patterns of actors involved in decision making. Two phases are distinguished, the negotiating phase, and the decision-making phase. During the first phase the agent's primary goal is to influence the policy position of other agents, whereas during the second phase the agent's goal is to find a solution (especially if there is a credible threat of a worse outcome). Convincing others is easier in a bilateral setting than in a multilateral one, and thus the decision-making phase is expected to result in an increase in multilateral interactions. To reveal the participation pattern of agents, we discuss methods for quantitative analyses of complex negotiations. A dynamic analysis of participation in multilateral negotiations gives insight into the continuity and change of participation during a negotiating process. The process of determining prices for specialized medical care in The Netherlands is used as a case study. The quantitative methods seem to be useful for analyzing such a complex case. Following our hypothesis, in the case study multilateral contacts are shown to be more frequent over time as compared to bilateral negotiations.