Scenario-based specifications offer an intuitive and visual way of describing design requirements of distributed software systems. For the communication paradigm based on messages, message sequence charts (MSC) offer a standardized and formal notation amenable to formal analysis. In this paper, we define shared variables interaction diagrams (SVID) as the counterpart of MSCs when processes communicate via shared variables. After formally defining SVIDs, we develop an intuitive as well as formal definition of refinement for SVIDs. This notion provides a basis for systematically adding details to SVID requirements.