2.1.8 Randomisation

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​​​​​​​​​​​​​​​​​​​​​​​A. Background & Definitions

Randomisation is a process of random assignment of experimental units to treatment conditions:

  • occurrence of one event should have no influence on the next event (independence principle);
  • randomisation sequence cannot be based on an easily memorizable and reproducible sequence (randomness principle).

Randomization serves three main purposes:

  • enables the application of statistical tests based on the central limit theorem;
  • prevents a potential impact of the selection bias due to differing baseline or confounding characteristics of the subjects;
  • supports the implementation of other means to reduce the risks of bias (such as blinding).

B. Guidance & Expectations​

Randomisation protocol should describe the following:

  • Type of randomisation (simple / unrestricted, block, stratified, etc.)
  • Block size (if applicable)
  • Stratification variables (if applicable)
  • Tools used for randomisation (including copy of a script if R, SAS or another similar script-based software is used)
  • Reproducibility of the randomisation protocol such as the seed of random number generator (if applicable)
  • Reference to the protocol followed (if applicable)
  • Methods to monitor / detect deviations from the protocol (if any)
  • If a decision is made not to introduce a proper randomisation protocol, the reasons should be discussed in a declaration justifying the decision to use pseudo-randomisation or simple interspersion methods.


  • Is pseudo-randomisation used instead of strongly recommended true randomisation?
  • Is there a risk that randomisation is introduced at allocation ​of subjects per experimental groups but is not maintained throughout the study conduct, outcome assessment and data analysis?​​​


  • To consider adding this subject to a training program for new employees or refresher training (if appropriate)
  • To assess the risks of cross-contamination when animals housed in the same cage are exposed to different pharmacological treatments
  • To check whether there are feedback channels installed so that your colleagues can identify, record and report errors and critical incidents related to this subject​ (if appropriate)​

C. Resources

Guidelines on reporting of randomization (in vivo research):

ARRIVE 2.0 ​​​​

Online tools to support randomisation:

Reading material:

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