Weight of evidence

Last update 9 October 2017

A variety of types of scientific evidence may be available when assessing the risks associated with the use of an industrial chemical. NICNAS scientists need to weigh up the quality of each piece of evidence (that is, they must consider the 'weight of evidence') in assessing a chemical's risks and recommending ways to reduce any unreasonable risks.

Different types of information about an adverse effect that may occur following exposure to a chemical (a 'toxicological hazard endpoint' such as skin irritation or eye damage) need to be reviewed and balanced in order to conclude whether a chemical is likely to pose an unreasonable risk to workers, the general public and/or the environment and—if so—what recommendations should be made to reduce that risk.

The use of a 'weight of evidence ' approach helps to reduce, refine or replace testing on animals, in line with international trends. NICNAS accepts evidence from tests on animals and humans ('in vivo'), from laboratory tests using cells and other biological tissues ('in vitro'), and from newer alternative methods such as computer-based models ('in silico').

NICNAS determines the quality of each piece of scientific evidence in terms of its reliability, relevance and fitness for purpose.  The quality of scientific evidence is primarily judged by whether it has been produced in accordance with guidelines endorsed by the Organisation for Economic Co-operation and Development (OECD), which are accepted by regulators internationally.

  • An in vitro assay result—from a test validated by the OECD and conducted in accordance with good laboratory practice—is an example of reliable information.
  • Information on an analogue1 chemical—that satisfies the requirements of the OECD Guidance on Grouping of Chemicals—is an example of relevant information.
  • Information from an animal study—which was conducted according to OECD guidelines—is an example of evidence deemed fit for the purpose of recommending GHS2 classification for the chemical.

Applying weight of evidence in assessing the risks of a chemical:

  • means giving more weight to information that is fully compliant with OECD guidelines—compared to those that are of lesser quality—in order to characterise a chemical's hazard(s);
  • provides a transparent mechanism to address inconsistencies between different pieces of evidence; and
  • allows NICNAS to consistently articulate uncertainties that reflect the different types and quality of available information.

In general, the greater the uncertainty, the more precautionary NICNAS needs to be in assessing the risks of a chemical, and recommending controls to mitigate (lessen) a risk.


1. A chemical for which acceptable evidence is available, whose properties and toxicological mode of action can be assumed to be similar to the chemical being assessed

2. Globally Harmonized System of Classification and Labelling of Chemicals