Existing research provide frameworks for analysing the rationale behind engineering methods and how this rationale matches the rationale of individual project members. As methods are used in groups, this raises questions about how to study method rationale on an aggregated project level. We propose an elaboration of method rationale theory to enable this type of analysis. We introduce the concept of collective method rationale together with metrics to capture this aggregated rationale. The conceptual work is implemented in a computerized tool, which enables analyses of collective method rationale in product development projects. These are the results of an action research project and we present a pilot test of the computerized tool to demonstrate the concept.