Weighting LCA results creates a single score

Weighting is the optional fourth and final step in Life Cycle Impact Assessment (LCIA), after classification, characterization and normalization. This final step is perhaps the most debated. Weighting entails multiplying the normalized results of each of the impact categories with a weighting factor that expresses the relative importance of the impact category. The weighted results all have the same unit and can be added up to create one single score for the environmental impact of a product or scenario. Simply put, weighting means applying a value judgment to your LCA results. It is a controversial step since the weighting factors you choose can influence the results and conclusions of your LCA.

• Related: The basics of the LCA methodology

Why use weighting?

Weighting is useful for several reasons. First, it presents LCA results as a single score, which allows you to easily compare the environmental impact of different products or scenarios. This facilitates decision-making since it is immediately clear whether a product’s impact is higher than, lower than, or similar to the alternatives. Second, weighting can be very helpful for communication purposes. It is much easier to explain a single score for environmental impact than it is to explain 3 to 18 different scores per product or scenario.

Figures 1 and 2 illustrate the difference between normalized and weighted results. In figure 1, products A and B have both been assessed for three impact categories, in each of their three life cycle stages. From the graphs, it is obvious that there are significant differences between the two products in each life cycle stage and each impact category, but at a glance, it is impossible to tell which product has the most impact overall. If nothing else, that judgment depends on which of the three impact categories is considered most important. In figure 2, on the other hand, products A and B can be compared directly. It is immediately obvious that product A has a larger environmental impact overall, and how much impact each product has in each life cycle stage.

Figure 1: Normalized results, showing results for multiple impact categories for product A and product B

Figure 2: Weighted results, showing a single environmental impact score for product A and product B

How are weighting factors determined?

Several impact assessment methods include one or more sets of predefined weighting factors, and each method has its own approach for determining them. It is important to be aware of the reasoning behind the weighting factors since they will have an important effect on the results of an LCA. Although each method uses a different approach for determining weighting factors, four basic categories can be determined:

• Distance to policy target. Some weighting factors are determined based on policy targets. If the goal for climate change, for example, is to reduce the national CO₂ emissions by 50% and the goal for acidification is to reduce the national SO2 emissions by 20%, the carbon footprint will get a higher weighting factor. An issue with this approach is that policy targets may not reflect how serious a problem actually is, since policy may be influenced by costs and other political considerations.
• Distance to scientific target. These weighting factors are based on the same approach as the distance to policy target, but use scientific targets instead. For example, there is a general consensus that 350 ppm is the maximum ‘safe’ level of carbon dioxide in the atmosphere with regard to climate change. These ‘safe’ levels are used as targets. The further away the current situation is from this target, the higher the weighting level. An issue with this approach is that not every environmental impact category has targets based on scientific data, and that the available scientific targets may not always be up to date.
• Monetization. This approach expresses the relative importance of an impact category in monetary value. This value can be based on the costs associated with preventing or repairing damage; one type of impact may be more expensive to prevent or fix than another. Another way to monetize an impact category is to measure how willing people are to pay to prevent a certain impact. For example, people might be willing to pay more to prevent impacts on human health than to prevent resource depletion. An issue with the monetization approach is that it inherently requires an answer to the question of how much damage is acceptable, and how much the value of human life is. This answer is at best subjective.
• Panel weighting. Perhaps the most straightforward form of determining weighting factors is to simply ask a large group of people for their opinion on what is more important. An issue with this approach is that the results can easily be skewed by the personal characteristics of the panelists. Different people have different levels of knowledge about environmental effects and different personal experiences with certain impacts. Beyond that, it is difficult to control the amount of explanation panelists are given and the effect of recent media attention on their awareness of certain issues.

If the weighting factors that are commonly available do not meet your needs, it is possible to create your own set of weighting factors, specific to your company or country.

To weight or not to weight?

As described earlier, weighting has many benefits. There is a risk, however, that people might consciously or subconsciously choose certain weighting factors to make the results reflect what they want to see. This risk makes weighting such a controversial step in LCIA. So should you use weighting or not?

Whether you should use weighting or not is closely related to your goal and scope. The leading standards for LCA, ISO 14040 and 14044, specify that you cannot use weighing for comparative assertions intended to be disclosed to the public. This makes sense since consumers are not LCA experts and will not know the implications of seeing a single, weighted environmental impact score.

In general, weighting is best used for internal decision making. In this scenario, weighted results allow you to focus on what is important for your company. Weighting is also a good option if you’re only investigating the impact of one product, without a comparison.

Conscious vs. unconscious weighting

An interesting aspect of the weighting debate is that, even if you do not deliberately apply weighting factors, you still apply value judgments whenever you make a decision based on multiple criteria. This is unavoidable. The only difference is whether the weighting occurs consciously, on paper, on unconsciously in your head. Since this is the case, perhaps it is better to be explicit and choose weighting factors based on clear reasoning. I’ll leave that choice up to the decision-makers out there.

Moving forward

An attempt to solve the weighting debate has been made in the form of endpoint methods, which combine a long list of midpoint indicators into three standardized endpoints, based on scientific factors. As a result, a decision-maker only needs to consider and apply a value judgment to three environmental impact indicators.

Whenever you are using weighted results, it is a good idea to also include characterized and normalized results in your communications, reports or presentations. This is a transparent approach that gives a more complete picture of the results and leaves less room for bias.

Read about other LCA steps, each explained by an expert:

• Characterization
• Normalization