## 3 The LCA Commons Approach to Distributed Data Development  
Deciding how to combine different models produced by different people at different agencies for different purposes into a single consistent data product remains a long-standing challenge for a distributed network of data developers such as the LCA Commons. The heart of the challenge is meeting two conditions for LCA data interoperability: distributed models are suitable for use together, and the information system can recognize and resolve the connections between them. The first condition, suitability for use, is most effectively assured with close coordination between parties during the goal and scope development of an LCA before data collection and modeling begins. The second condition, an information system to resolve connections, requires all parties adopt consistent technologies and standards. The details and challenges of connecting distributed datasets is illustrated by the LCA concept of the product system. A product system is a set of processes or subsystems, connected to each other through the exchanges of valuable materials, energy, or services. Each process or subsystem represents an LCA model, and usually represents a specific economic activity or production process. A complete cradle-to-grave LCA will have a product system including the full production supply chain, use, and end of life. It is these different components of a product system that are almost always developed by different parties with specific expertise. Product systems in LCA are often modeled as a graph, with a set of nodes representing processes or activities, connected by edges representing the exchange of valuable material or energy Figure 2. For the LCA Commons, the connection between processes happens through the flow object. The flow represents a specific quality of material or energy that can be exchanged between processes or firms. Examples of flows include: Electricity, AC, 120V, and Containerboard, 100% recycled, at mill. A set of separate, but related, processes combines to form a single model or dataset, perhaps within a single industry, connected by the exchanges of valuable flows. Purchased, sold, imported, or exported material and energy leaves the system boundary of a specific industry dataset, and can connect to the providers and consumers of these material and energy flows. The ability to connect these material energies depends on consistent quality of the flows between products and demands of the connected process models. The graph of connected processes can extend to the industry specific models produced by different organizations working within the LCA Commons. Different classes of activities produced by different participant organizations within the LCA Commons include electricity, fuels, transportation, and waste management. Producing a reasonably accurate LCA model requires a combination of suitable independently developed datasets with consistent scopes, goal definitions, and data collection requirements. This fitness-for-purpose of a dataset as a component of a larger distributed model is either assured in advance through active coordination between the research teams developing the datasets or must be determined after the development by reviewing a dataset's metadata and documentation, and if possible, interviewing the data developers. The LCA Commons engages in both types of data interoperability activities, coordinating to develop background datasets that can interoperate between each other and serve as a background data product for a range of applications. The LCA Commons also documents and formats datasets with high reuse potential to accurately determine so their fitness-for-purpose for new applications may be accurately determined in the future. An information system that can recognize and resolve flows is the second condition for connecting decentralized LCA models. The LCA Commons uses common flows as the method to connect different process models. For example, a process that uses electricity may have Electricity, AC, 120V as an input.