A weighting factor is a characterisation factor based solely on value choices describing the relative contribution to an Impact category endpoint from other (intermediate) impact category endpoints. This distinguishes weighting factors from other characterisation factors, which may be derived from physical, chemical, or biological models of environmental cause-and-effect relationships.
Whilst ISO 14040 defines characterisation factors more broadly as factors that convert life cycle inventory analysis results to impact category indicators, weighting factors occupy a unique position in Life Cycle Impact Assessment (LCIA) methodology. They are exclusively grounded in normative judgements about the relative importance or severity of different environmental impacts rather than in scientific models or empirical observations.
The application of weighting factors represents the optional weighting step in LCIA, which ISO 14044 explicitly states "shall not be used for comparative assertions disclosed to the public." This restriction reflects the inherently subjective nature of weighting factors, as they embody societal, ethical, or stakeholder preferences about which environmental problems matter most. For instance, a weighting factor might express whether climate change should be considered more or less important than ecosystem quality degradation or human health impacts.
Weighting factors enable the aggregation of multiple impact category endpoints into a single score, facilitating decision-making when trade-offs exist between different environmental impacts. However, because these factors reflect value judgements rather than scientific consensus, their use requires transparency about the value choices embedded within them. Different stakeholders may legitimately apply different weighting factors based on their priorities, geographical context, or ethical frameworks, leading to different conclusions from the same inventory data.
