Chapter 4 Data Quality

Source inventories and release estimates reported under Article 15 should be:

  • Reliable,
  • Consistent over time,
  • Comparable between countries,
  • Transparent, and
  • Complete.

Reliable inventories entail coherent application of internationally acknowledged methodologies such as the Toolkit and the use of best available national information.

To achieve consistency over time, the same approach should be used over time to establish consistent time trends.

To ensure comparability between countries, all countries should report according to the same source groups and source categorization.

For transparent estimates, the approach, methodology, information, and assumptions used should be clearly described, documented and archived to facilitate inventory updates in the future.

For complete release inventories, all relevant source categories, all sources within those categories and all relevant release vectors have to be considered in the whole country. The inventory should also include information on source categories that do not exist or are not operational in the country during the reference year.

4.1 Quality Assurance and Quality Control (QA/QC)

The following quality assurance and quality control measures (QA/QC) should be applied to ensure that the source inventory and release estimates meet the quality criteria described above:

Activity Rates

  • Align the unit of the activity rate with the unit of the emission factor.
  • Pay attention to orders of magnitude while recalculating the activity rates and applying the emission factors.
  • Explain clearly and completely all assumptions made in filling gaps in activity rates (see also “completeness”).
  • Explain clearly and completely the process of classification of sources and the way activity rates were derived.

Emission Factors

  • The Toolkit expert group is mandated to evaluate all emission factors that are or will be included in the Toolkit to determine that they are scientifically sound.
  • National emission factors should only be derived from measured data of adequate quality e.g. the application of standard sampling and analytical methods; proven laboratory experience and good documentation are pre-requisites of high quality data.
  • The classification of sources and choice of emission factors have to be explained, documented and archived.
  • Consideration has to be given to units and orders of magnitude.

Completeness of Data

  • Whenever practical and appropriate, individual plant questionnaires may be used to gather information for large point sources.
  • Questionnaires provide useful information for the classification of plants and selection of emission factors. Since the return rate of the questionnaires is likely to be low, incomplete information-data gaps- will need to be covered by making assumptions about certain sources, where no specific information can be collected. Approaches will vary, but all assumptions should be clearly described in order to facilitate inventory updates in the following years or revisions in light of improved information.
  • To determine complete activity rates, a combination of questionnaires (for large point sources) and national statistics should be used.
  • When reporting the inventory results, it should be distinguished between “not applicable” e.g. the source category does not exist or is not operational in the country, and “not estimated” e.g. the source category is relevant but there was no sufficient information to estimate the releases.

Additional Considerations for Assessment of Inventory Results

  • Compare national inventory results with results from other countries (see Annex 7).
  • Compare national inventory results across different time periods: differences have to be justified, documented and logically explainable.
4.2 Data Quality

Possibilities of indicating the confidence in the data used to generate emissions estimates may be as follows:

  • Reporting of ranges (gives a good indication of confidence in data, however it may create problems while summarizing releases from more countries, therefore suitable only for reporting at the national level);
  • Simple qualifiers, i.e., data quality codes “high”, “medium”, “low” as outlined in Annex 8.

Qualifiers may be applied to both emission factors and activity data to discern the overall confidence in the inventory’s results. Quality ranks are assigned to default emission factors by the Toolkit expert group to enable informed use of the methodology to assess PCDD/PCDF releases. Technical annexes provide complementary information on how default emission factors were derived to ensure that they are applied with the specified level of confidence only when matching a specific situation. Furthermore, guidance on assessing data quality criteria for calculating activity rates is provided in Annex 8.

More details on data quality and inventory QA/QC can be found in Part II Default Emission Factors.