4. Proposals involving animal use - MRC


4.3 Experimental design, avoidance of bias and statistical considerations

There is a wide range of designs and approaches to animal experimentation that are appropriate depending on the objectives of the research proposal. In all cases, the MRC expects that researchers provide well justified information in their applications concerning the experimental design and its suitability to answering the research questions posed.

While we recognise that there are ethical imperatives to reduce the number of animals used, it is also unethical to conduct a study that, because of its limited size, has inadequate statistical power to robustly answer a research question. Applicants should therefore provide adequate justification for their choice of design and numbers of animals and interventions. It is important that adequate information is given concerning methodological issues including – but not restricted to – the following:

  • the avoidance of bias, for example blinding of observers assessing outcomes to the group allocation in a randomised design
  • how randomisation will be carried out if used, or why it is not appropriate if it will not be used
  • a clear definition of the experimental unit in the analysis and the implications thereof – that is, there is a difference between N samples from one animal, as distinct from one sample from each of N animals, or combining samples from multiple animals
  • a principled justification of the adequacy of the numbers of animals to be included so as to be able to minimise the likelihood of spurious results due to the play of chance alone
  • where animals are used in multiple types of experimental approaches within a single application, for example for tissue supply, pilot experiments or more defined preclinical studies – exemplars for these types of experiment should be provided
  • the sex of the animals used, and if only a single sex is proposed, justification for why the study fits one of the exemption categories outlined by MRC
  • the number of different time points at which measurements will be made on each animal
  • a description of the statistical analysis methods that will be used, explaining how they relate to the experimental design and showing that they are appropriate for the types of data that will be collected
  • an indication of the number of independent replications of each experiment to be performed with the objective of minimising the likelihood of spurious non replicable results. If there are no plans for studies to be independently replicated within the current proposal then this will need to be justified.

Examples of the level of detail and type of information required can be found on the UKRI website.

In addition, the NC3Rs have developed a free online tool to guide researchers through the design of their experiments, helping to ensure that they use the minimum number of animals consistent with their scientific objectives, methods to reduce subjective bias, and appropriate statistical analysis.

The Experimental Design Assistant helps applicants build a machine-readable diagram representing their experimental plan, following capture of their methodology, and allows the applicant to then generate a PDF report that provides a transparent description of the experimental design in a standardised format. This tool can assist applicants in the design of experiments using both sexes of animal, tissue or cell.

Applicants are encouraged to consider embedding the summary diagram of this tool into their one-page reproducibility and statistical design annex, where appropriate.

Last updated: 16 June 2022

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