Definition of uncertainty
Uncertainty is the estimated amount or percentage by which an observed or calculated value may differ from the true value. Uncertainty is usually expressed as a range of values and the probability that the true value falls within that range. For example, the uncertainty for a company’s annual GHG emissions may be expressed as +/- 30%, at the 95% confidence level (i.e. there is a 95% chance that the true value is within a range 30% above or below the calculated value for annual emissions).
Using the uncertainty assessment in Ecometrica Sustainability
The uncertainty assessment in Ecometrica Sustainability gives an indication of the relative uncertainty associated with activity data (depending on whether the data is actual or estimated) and the emission or conversion factors (depending on whether the factors have more or fewer built-in assumptions), and can be used to reduce uncertainty by encouraging reporting companies to improve the type and quality of data collected. The uncertainty score for an assessment can be reduced by:
1. Providing actual data rather than estimated data.
2. Providing data for the whole assessment period rather than extrapolating data.
3. Increasing the level of detail provided for emission sources (e.g. specifying “small”, “medium” or “large” vehicle rather than “average”).
4. Providing transportation data in energy, mass or volume units for fuel consumed rather than providing data for distance travelled (as calculating emissions from distance travelled requires an assumption about fuel economy).
5. Providing energy consumption data in energy, mass or volume units rather than providing data for energy costs or floor area occupied (as calculating emissions from cost data or floor area requires additional assumptions and conversions).
6. Providing waste data in mass units rather than by volume or number of bin bags (as calculating emissions from volume data requires an assumption about the density of waste).
The uncertainty assessment is best used as a tool for improving the type and quality of data provided rather than as an objective measure of the amount by which true GHG emissions differ from estimated GHG emissions.
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