Organising participation of multiple stakeholders is nowadays a widespread request in decision processes, especially for organisations managing environmental risks. Therefore, analysts delivering decision support are expected to provide decision makers with scientifically sound and practically realisable approaches regarding this issue. One of the main challenges in dealing with participation is the definition of the organisation, the so called participative structure, through which stakeholders will contribute and interact during the decision process. Who should participate when and according to which rules are the main questions to be answered. Stakes associated to this challenge are of extreme importance for decision makers since decision legitimacy and acceptance strongly relies on the ability to demonstrate a real transparency and information disclosure during the whole decision process. This paper proposes the iterative comparison approach as a new and original frame to be used by an analyst supporting a client dealing with such questions. Through an unambiguous definition of cognitive artefacts to be constructed when designing participative structures, this paper provides a clear framework that organises an analyst intervention in participative contexts. Furthermore, it offers the opportunity to design tailored participative structures that integrate context specificities in one hand, and satisfies quality criteria being fairness, competence and efficiency on the other hand.