Environmental News, 70 – The Product, Functional Unit and Reference Flows in LCA

The Product, Functional Unit and Reference Flows in LCA

5 Uncertainty

5.1 Causes of uncertainty
5.2 Minimising uncertainties

5.1 Causes of uncertainty

In the procedure outlined in Chapter 3, the following uncertainties may need consideration:

Uncertainties from conceptual errors (assuming importance of irrelevant information and disregarding relevant information) throughout the procedure.
Uncertainties in the test conditions and measurement methods for determining reference flows.
Uncertainties in determining the precise nature of the interaction with other product systems.

Uncertainty from conceptual errors

Conceptual errors may arise when determining the relevant product functions:
Overlooking an obligatory product property may lead to assuming a substitution that cannot take place in practice. In parallel, assuming a product property to be obligatory although it is not, may lead to a disregard for a substitution that actually will take place. The size of the uncertainty will be determined by the difference between the market segments with respect to the product property in question.

Office chair example

Office chair

It is a crucial assumption for the study of the office chair that the more advanced seat and armrest adjustment functions are obligatory properties. If these functions were in fact not obligatory, several other alternatives might have been included in the comparison. Omitting the mechanics providing the seat and arm rest adjustment functions could lead to significant savings in environmental impact, e.g. around 40% of the photochemical ozone formation from the product system of the chair in figure 4.

In a similar way, conceptual errors may arise when determining the relevant market segment:

either when assuming a possibility of a substitution that can not take place in practice (e.g. assuming that a labourers' chair could be sold as a computer workstation chair). The size of this uncertainty is determined by the difference between product alternatives in the different market segments.
or by disregard for a substitution that actually will take place
(e.g. disregarding that a German chair could be sold in Denmark or that two computer workstation chairs could substitute each other). The size of this uncertainty is determined by the variation within the market segment, which means that it will typically be lower than the above difference between market segments.

Also in determining the relevant product alternatives, conceptual errors may be the disregard of relevant alternatives, or the inclusion of irrelevant alternatives. The size of this uncertainty depends on the difference between product alternatives within the identified market segment. Therefore, these uncertainties are typically lower than the above uncertainties related to differences between market segments.

When determining the reference flows, conceptual errors may be a similar disregard for properties which determine differences in the amount of product consumed, or inclusion of irrelevant properties.

Uncertainties in measurement of relative performance

Even when the correct properties have been identified, uncertainty may be caused by bias in the test conditions and uncertainty in the measurement methods used to determine the relative performance of the studied products.

Example:
In a comparison of lighting alternatives, the lifetimes of the bulbs may be assessed to 10000 and 5000 hours respectively.The lifetimes are assessed with an uncertainty of +/- 10%.The test conditions have no bias towards particular bulbs.

Also in determining the interaction with other product systems, uncertainties may arise from conceptual errors, i.e. disregard of an interaction with another product system, or assuming a non-existent interaction.

Uncertainty from system expansion

Furthermore, there may be uncertainties from the procedure of determining the additional processes to be included in the product systems. These uncertainties are described in more detail in the technical report "Market information in LCA."

5.2 Minimising uncertainties

To minimise the uncertainties described above, the following precautions should be taken:

To minimise the probability of conceptual errors and bias, stringency should be observed throughout the procedure, especially in collection, documentation and interpretation of market data and data on functionality of the products. An independent critical review procedure can contribute to reduce this source of error.
Special caution should be observed with respect to the largest sources of uncertainty, i.e. the market segmentation. Conservative assumptions (i.e. assuming no substitution) will typically give less uncertainty than liberal assumptions (i.e. assuming substitution), but when in doubt it is preferable to report the uncertainty and include several alternative scenarios to reflect the limits of knowledge. The extent of such alternative scenarios depends on the needs of the target group for the study.