Weight of evidence

The weight of evidence approach means that you use a combination of information from several independent sources to give sufficient evidence to fulfil an information requirement.

This approach is beneficial when,

  1. the information from a single piece of evidence alone is not sufficient to fulfil an information requirement. This could be, for example, due to clear deficiencies in one of the existing studies.
  2. individual studies provide different or conflicting conclusions.

The weight you should give to the available evidence depends on factors such as the quality of the data, consistency of results, nature and severity of effects, and relevance of the information.

The weight of evidence approach requires use of scientific judgment and, therefore, it is essential to provide adequate and reliable documentation.

As a general principle, the more information you provide, the stronger your weight of evidence is. Make sure that you present the information in a structured and organised way and take into account the robustness and reliability of the different data sources to support your justification. For example, in vivo and in vitro data carry more weight in the decision than a QSAR or other computational methods.

To start building your weight of evidence case, gather all information from all possible sources including:

  • published literature
  • read-across from chemical analogues
  • (Q)SAR predictions
  • data from existing studies
  • in vitro studies
  • epidemiological data/human experience.


  • A proper weight of evidence approach includes, as a minimum, two separate study records for the property - also when using textbook values. One single value from a secondary data source is not sufficient as a weight of evidence.
  • Choose an expert who has expertise in the relevant properties and study methods. This expert will need to assess the reliability, relevance, adequacy of the available data and judge whether the combined evidence is enough to draw a conclusion about the property or effect of the substance.
  • Make this expert judgement transparent and understandable by documenting all information used, all steps carried out in the evaluation process and all conclusions drawn.
  • Provide a scientific justification and documentation of the underlying evidence.
  • Provide all information that is relevant in your dossier. ECHA or the other authorities do not have the same detailed knowledge about your substance as you.