Further theory on avoiding uncertainty when defining the functional unit
In some instances, two products are so closely linked (or complementary) that the separation of some of the processes in their life cycle may lead to an increase in uncertainty. If all the analysed product systems provide the same amount of such linked products, this additional uncertainty may be avoided by including both products in the functional unit – i.e. expanding the functional unit. Take the example of beverage containers, where a separation of the beverage and container would lead to an uncertainty in determining how much of the transport to allocate to the container. This uncertainty can be avoided by expanding the functional unit to include the transport of the beverage. Since the functional unit for the containers will typically be related to the volume of beverage protected and distributed, the expanded systems are still comparable.
When presenting the outcome of an LCA, it is important to communicate that the result, as e.g. presented in the inventory tables, does not represent the environmental exchanges of the functional unit in itself, but the environmental exchanges resulting from adding or subtracting one functional unit compared to the other systems studied, or by fulfilling the performance described by the functional unit compared to not fulfilling this performance.