How To Randomise
Randomisation is the gold standard for clinical trials so that we avoid bias. Randomisation ensures the treatment allocation is provided by chance and not by choice, either on the part of the clinician or the patient/recipient. To allow for randomisation you need a treatment group and a control group. The control group can be standard care, a sham procedure or in the event of no recognised standard of care, a placebo maybe considered. Consultation with a statistician is highly recommended.
The method of randomisation can vary:
How to generate a randomisation sequence
Simple randomisation is analogous to repeated fair coin tossing
How to use a random numbers table:
Where to find a random numbers table:
“No other allocation generation approach, irrespective of its complexity and sophistication, surpasses the unpredictability and bias prevention of simple randomisation.” Schulz & Grimes 2006
Problems with simple randomisation
Blocking (random permuted blocks)
It is one form of restricted randomisation which aims to create unbiased comparison groups of about the same size throughout the trial.
Blocking Example: Block size of 6
Block 1: AAABBB
Block 2: BBBAAA
Block 3: AABBAB
Block 4: BBAABA
Block 5: ABABAB
Block 6: BABABA
Block 7: ABAABB
Block 8: BABBAA
|Choose a block at random and the first 6 treatments are allocated according to the permutations in that block. Then a new block is chosen at random and the next 6 treatments are allocated according to that block. Keep going until the required sample size is recruited.|
Stratification involves dividing the sample to be studied according to prognostic factors.
See related toolkit – Randomisation services in Australia
Meinert CL. Clinical Trials. Design, Conduct and Analysis. New York: Oxford University Press, 1986. Schulz K, Grimes D. The Lancet Handbook of Essential Concepts in Clinical Research, Philadelphia: Elselvier Ltd. , 2006
These materials are based on content originally provided by the WOMBAT collaboration. The WOMBAT Collaboration was formed to promote and support high quality randomised clinical trials in the perinatal area in order to improve the health and wellbeing of women and their children (2005 – 2010).
This toolkit was prepared by Rebecca Tooher (WOMBAT) and Philippa Middleton (WOMBAT). Updated by Lucille Sebastian (NHMRC CTC) and Hala Phipps.
Last revised: 9 December 2015.