The importance of categories is usually addressed by using “weighing factors”. For example, the EPA’s BEES tool uses weighing factors to come up with a single score. See this BEES User Manual for how the tool weighs its impact categories (Table 2-8, also screencaptured here), and how the weights were created. As mentioned before though, weighing is generally based on someone’s consensus and is subjective to what people perceive as important to them. Weighing can be helpful for putting things into perspective but should be taken with a grain of salt.
