Published on November 16, 2015

Every researcher has heard about randomisation, but what is it for and how is it used? In this article we will explain the concept and how it can be applied in practice.

What is a clinical trial?

Randomization is most often found in clinical trials. Clinical trials are performed to provide insights about the efficacy of an intervention (e.g. treatments or changes in behaviour). Usually a trial aims to compare two or more groups of patients on a specific outcome, for example the survival rate.

Why do we use randomization?

“The RCT is considered the golden standard of clinical research”

The randomized controlled trial (RCT) is universally considered to be the golden standard when conducting clinical research. In an RCT, the allocation of patients to a specific intervention group is done by way of randomization. The groups of participants are kept as similar as possible, with one exception: the intervention itself. By keeping the differences between the intervention groups as small as possible, you reduce the chance that a measured effect on the selected outcome is due to bias; for instance, only very sick patients received the intervention.

Double-blind randomization definition

“Double-blind randomization ensures researchers and patients do not influence the trial”

An extension to the normal RCT, often used in research using placebos, is the double-blind randomised controlled trial. The term double-blind refers to the fact that both the researcher and the patient do not know which randomization group the patient was allocated to. The advantage of this blinding is twofold: the researcher cannot affect the outcome of the study by treating patients in the intervention group differently and the patient is not influenced by knowing whether he or she received the placebo.

How does randomization work?

The random allocation of study participants to intervention groups can be described by using a classic probability case: the vase with marbles everyone remembers from high school. By putting in an equal number of marbles for each intervention group (red or blue) and (blindly) taking out a marble for each patient, you can ensure that the intervention allocation is performed at random.

Which randomization method does Castor use?

A potential problem with the retrieval of marbles from the vase is that the researcher must know how many patientsknikkers are going to be included (how many marbles should be put in the vase). And with that information, the researcher can infer what the colour of the last marble will be, potentially breaching a double-blind randomization procedure and influencing the research results.

“Castor uses a variable block randomization method”

To overcome this problem, Castor uses a variable block randomization method. With this method, Castor puts a small but equal number of marbles in the vase (a ‘block’) for each intervention group. These blocks consist of either 2, 3, or 4 marbles. This way the researcher does not know how many marbles are in the vase at any given moment, and thus cannot predict the next randomization.

How Randomisation is done with castor
Online randomization with Castor

Getting started yourself

Are you using Castor already and do you want to use randomization in your study? You can easily enable randomization on your Settings screen. For more information you can check our knowledge base.

Haven’t tried Castor yet but looking for an online data capture system that allows you to easily integrate randomization in your study?

Start your randomized trial

This is the first in a series of blogposts about research methodology. If you’re interested you can follow our Twitter or Facebook accounts to receive updates.

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