When an LCA is used to compare two or more products, the basis of comparison should be equivalent use, that is, each system should be defined so that an equal amount of product or equivalent service is delivered to the consumer. For example, if bar soap were compared to liquid soap, the logical basis for comparison would be an equal number of handwashings. Another example of equivalent use would be in comparing cloth diapers to disposable diapers. Cloth diapers need to be changed more frequently, or they are doubled whereas disposables are not. Thus, more cloth diapers will be used. In this case, a logical basis for comparison between the systems would be the number of diapers used over a set period of time.
Equivalent use for comparative studies can often be based on volume or weight, particularly when the study compares packaging for delivery of a specific product. A beverage container study might consider 10001 of beverage as an equivalent use basis for comparison, because the product may be delivered to the consumer in a variety of container sizes having different life-cycle characteristics.
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