2.1.8 Randomisation

​​​​​​​​​​​​​​​​​​​​​​​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.

RISK ASSESSMENT

• 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?​​​

PLEASE DO NOT FORGET

• 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:

• NC3Rs’ Experimental Design Assistant

• QuickCalcs​

• Sealed Envelope​

• RandoMice software - [read] - [download and install]

Reading material:

• Handbook of Experimental pharmacology chapter on randomization and blinding

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