Clinical trials glossary

Use our glossary of some common terms often used in clinical trials so that you're able to be as informed as possible throughout your clinical trial.


Keeping to the treatment given in a clinical trial.

Adverse events (AE)

Unwanted effects and symptoms that may or may not be caused by the trial treatment such as nausea, pain, low blood counts. If the symptom or problem is caused by the treatment it is known as a side effect.


Audit in healthcare is a process used by health professionals to assess, evaluate and improve care of patients in a systematic way. Audit measures current practice against a defined (desired) standard.

Blinding and double blinding

Blinding means that the people having trial treatment do not know which treatment they have been given. Double blinding means that the people on treatment and their doctors also don't which treatment is being given (until the end of the trial when all the data has been collected).

Closed to recruitment

This is when no more new people can join the trial. However, existing participants continue to be followed up as part of the trial. When all the trial data needed is collected the whole trial is then closed.

Cross-over trial

A trial where the treatment changes partway through the trial by participants crossing over from one treatment group to the other.

Data monitoring committee

This is an independent committee of experts that oversee the trial’s progress. They may advise changes to the trial or stopping it if they see evidence of unexpected problems for the participants. Also in cases where one treatment is obviously better than another, they can advise that a trial is stopped.

Disease progression

The course of a disease over time.

Eligibility criteria

The clear rules about who can or cannot join the trial, that is whether a person is eligible or not.  The rules are known as inclusion criteria (those that can join) such as having a particular type of disease or stage of disease and exclusion criteria (those that cannot join) such as being over or under a certain age or being pregnant.


The signing up of people to join a clinical trial. This involves matching a potential participant to the eligibility criteria of the trial and going through the informed consent process.

Ethics committee

A committee of a mixture of healthcare professionals and lay people who review funded clinical trials. This ensures the trial is run to appropriate ethical standards. The trial must be approved by an ethics committee before people are recruited to the trial.

Evidence base

The best available scientific research available about a disease or health problem. This is used to make decisions about the best treatment to give.

Feasibility study

Feasibility studies are used to determine whether an intervention is appropriate for further testing. They enable researchers to assess whether or not the ideas and findings can be shaped to be relevant and sustainable.

Good Clinical Practice (GCP)

This is an international quality standard for the conduct of clinical studies. Randomised clinical trials are required by law to conform to GCP.

Health economics

This is when some trials look at and compare the cost of effective treatments.

Informed consent

This is when a person who is being asked if they want to join a clinical trial agrees to take part of their own free will. They must be given all the information about the trial, usually both in verbal and in written form. They must also have the chance to ask questions and have time to think about their decision before giving their consent to join the trial. Usually a consent form is signed and dated by both the participant and the trial doctor.

A person is still able to withdraw anytime from a trial even after giving informed consent, without giving a reason and without it affecting their healthcare.

Interim analysis

An analysis of trial data done at an appropriate time before the end of the trial to check on progress of the trial.


This is the treatment or method that is being looked at and tested in the trial, that is hoped to be an improvement on the usual standard treatment.


This is a way to combine the numerical results of several trials about the same treatment to provide an 'average' estimate of the effects of the treatment.

Observational study

Observational studies are ones where researchers observe the effect of a risk factor, diagnostic test, treatment or other intervention without trying to change who is or isn’t exposed to it. 

Open label trials

This means that the people on the trial and their doctors know which treatment they are having.

Open to recruitment

A trial that can still have more people joining.

Outcome measures

The measures of health and wellbeing such as the response to treatment or the recurrence of disease.


A person that takes part in a clinical trial

Patient information sheet (PIS)

This describes all the relevant trial-related information in detail. It needs to be read and understood by the potential participant before informed consent is given.


This is a dummy treatment designed to have no effect. It looks, smells and tastes like the treatment being tested. In this way the trial participants do not know if they are taking the dummy treatment or real treatment (see 'blinding').


This is a measure to prevent a possible health problem, such as a vaccination or taking anti-sickness treatment to prevent feeling sick before the sickness starts.


This is the overall plan with the rules and information for the clinical trial. It provides information and background on the disease and the treatments being tested and why they are being tested. The protocol also states the eligibility criteria for participants and the timetable of the trial. Protocols have to be approved by an ethics committee.

Quality of life

This looks at and measures the impact of the trial treatment on people's quality of life. It may check on tiredness and mood, sense of wellbeing, activity levels and sleep patterns.


A computer will decide at random which treatment a trial participant will have, so that each person on the trial has the same chance of getting the treatments that are being compared and tested. It ensures that the groups of people being compared in a trial are as similar as possible apart from the treatment they have. This means that any differences in outcomes are more likely to be due to the treatments being compared.

Randomised controlled trial (RCT)

This is a trial that has a treatment group (people getting the treatment being tested) and a control group (people getting either a placebo or the usual standard treatment). People are placed at random into the different groups, which means there is a fair comparison being made between the trial groups.

Serious adverse events (SAE)

These are adverse events, which may or may not be caused by the trial treatment, that are regarded as more important and serious. Such as an unexpected stay in hospital or a life-threatening problem that happens while participating in a trial. All SAEs are reported to the MHRA (Medicine and Healthcare Products Regulatory Agency).

Side effects

If the symptom or problem is caused by the trial treatment, it's known as a side effect.


The organisation responsible for the trial.

Standard treatment

An approved and widely used treatment that is considered to be effective for a particular disease or condition.

Systematic review

A collection of evidence from all the global research studies relevant to a particular research area. It gives a reliable way of reviewing the evidence. When researchers combine the numerical results of these trials and compare them, this is called a meta-analysis.

Trial arm

This is one of the groups within a randomised controlled trial. One or more of the groups are treatment arms and one is usually a control arm. The people in the control arm get either a placebo (dummy treatment) or the usual standard treatment.

Trial Management Group (TMG)

This is a committee of trial researchers responsible for the trial being set-up, the day-to-day running of the trial and the release of any trial results or publications. It includes doctors, nurses, trial and data managers, statisticians and patient and public representatives.

Trial phases

Clinical trials are conducted in several phases.

  • Phase I: tests treatment safety in a small group of people
  • Phase II: looks at the effectiveness of the treatment being tested, and usually has a larger group of people
  • Phase III: compares two or more treatments and checks on side effects; the trial results are used to make recommendations about the licensing and use of the treatment
  • Phase IV: a post-marketing trial (when the treatment is already in use) that collects wider information on use of the treatment.