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Randomisation is a method of allocating or selecting without using any system. It is purely random. In clinical trials, participants are generally allocated to different arms of the trial (for example, to receive the study medicine or the placebo) randomly. This is a key part of the randomised controlled trial (RCT).

Randomisation in clinical trials means that each participant has an equal chance of being in any of the arms of the trial. It is an important method to reduce the risk of bias in the outcomes of the trial.

Randomised clinical trial

A randomised clinical trial is one that uses randomisation when allocating people to different arms of the study. For example, in a trial comparing a new medicine with a placebo, each person has an equal chance of being allocated to the medicine or to the placebo group.

Randomised controlled trial

A randomised controlled trial is a trial in which people are allocated at random (by chance alone) to receive one of several clinical interventions such as a new medicine. One of these interventions is the control group, for example a placebo may be given, no intervention at all, or the current best treatment available. This study is one of the simplest and most powerful tools in clinical research.

Randomised participants

Participants in a trial who have been randomly (by chance) assigned to one intervention arm or another of that trial. Practical considerations, such as missing data over time, may lead to some participants not being included in the final analysis.

Rare disease

A rare disease, also referred to as an orphan disease, is any disease that affects a small percentage of the population. Rare disease are commonly defined as life-threatening or chronically debilitating diseases which are of such low prevalence (fewer than 1 in 2,000 people) that special combined efforts are needed to address them. Diseases that are statistically rare, but not also life-threatening, chronically debilitating, or inadequately treated, are excluded from this definition. A disease may be considered rare in one part of the world, or in a particular group of people, but still be common in another.

Most rare diseases are genetic so that most people show symptoms from childhood (although some rare diseases only become apparent later in life).

Reassortant vaccine

A subtype of attenuated vaccines containing genetic material from at least two different strains of the same pathogen.

Recombinant DNA

Recombinant DNA, often shortened to rDNA, is an artificially made DNA strand that is formed by the combination of two or more gene sequences from different species. Recombinant DNA is engineered specifically for a purpose. For example, recombinant DNA can be used to change the genetic makeup of a bacterial cell in order to make it produce insulin. Recombinant genes, and the recombinant proteins they produce, have become widely used in medicine. They offer a novel method of managing some health conditions, such as the use of recombinant insulin in diabetes.

Recombinant Gene

A recombinant gene is one which has been changed by the addition and/or removal of some of its sequence. This can happen naturally or may be done artificially in the laboratory.

Natural recombination happens when the chromosomes in parents interact and exchange genetic material so that their offspring inherit combinations that are different to both parents.

Recombination is one of the important causes of genetic diversity between generations and individuals.

Recombinant vaccine

A vaccine using an antigen produced using recombinant DNA technology.


Recruitment is the process used by investigators to enrol people (participants) into a clinical study. Recruitment is based on the inclusion and exclusion criteria that are documented in the study protocol.

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