Research · Glossary

Clinical trial glossary

Plain-English definitions of the terms you'll see across this site and on official trial records. Hover or tap any underlined word in a trial summary for a quick definition.

A

adverse event: Any unwanted medical problem that happens to someone in a trial. It might be caused by the treatment or by something else.

B

biomarker: A measurable sign in the body (like a blood test result) that gives information about a disease or treatment.
blinding: Hiding from participants or researchers which treatment people are getting, to reduce bias in the results.

C

cohort: A group of people in a study who share something in common, for example all starting the same treatment together.
control group: The group in a trial that does not receive the new treatment. They might get a placebo or the current standard treatment, used as a comparison.
crossover: A trial design where each person tries both the new treatment and the comparison (like a placebo) at different times.

D

dose: How much of a treatment you take, and how often.
double-blind: Neither the participants nor the researchers know who is getting the real treatment until the trial ends. This helps stop bias.

E

eligibility criteria: The list of conditions you must meet (or not have) to take part in a trial, such as age, health, or other treatments.
endpoint: A specific event or measurement the trial is designed to look at, such as survival, recovery, or change in symptoms.
enrollment: The number of people taking part in a trial, or the act of joining one.
enrolment: The number of people taking part in a trial, or the act of joining one.
ethics committee: An independent group that checks a trial is safe, fair, and ethical before it can start.
exclusion criteria: The things that mean you cannot take part in a trial (for example, certain other illnesses or medicines).

G

GP: A General Practitioner — a family doctor who treats most everyday health problems.

I

inclusion criteria: The things you must have or be to join a trial (for example, a certain age or diagnosis).
informed consent: A process where the trial team explains the study to you and you decide, without being pressured, whether to take part.
institutional review board: An independent committee that reviews trials to make sure they are safe and ethical for participants. Also called an ethics committee.
intervention: The treatment, test, or change that the trial is studying — for example a new medicine, a procedure, or a lifestyle change.
interventional: A study where researchers give a treatment (like a drug or device) and look at the effect.
investigator: The doctor or scientist in charge of running a trial at a particular site.

N

NHS: The National Health Service — the publicly funded healthcare system in the UK.

O

observational study: A study where researchers watch what happens to people but do not give them a new treatment.
open-label: Everyone (participants and researchers) knows which treatment is being given. The opposite of blinded.

P

phase 1: Early trials, usually in a small group of healthy volunteers, to check if a treatment is safe and find a sensible dose.
phase 2: Trials in a larger group of people who have the condition, to see if the treatment works and to learn more about side effects.
phase 3: Large trials comparing the new treatment with the current best treatment or a placebo, often involving hundreds or thousands of people.
phase 4: Studies done after a treatment is approved, to watch for long-term side effects and check how it works in everyday use.
placebo: A harmless pill, liquid, or procedure that has no active treatment in it. It looks like the real treatment so researchers can fairly compare results.
primary outcome: The main result the trial is set up to measure — the question it is really trying to answer.
protocol: The detailed plan for the trial. It explains who can join, what will happen, and how the results will be measured.

Q

quality of life: A measure of how a person feels day-to-day — their wellbeing, comfort, and ability to do normal activities.

R

randomised: When people in a trial are put into different treatment groups by chance (like flipping a coin), so the groups are fair to compare.
randomization: When people in a trial are put into different treatment groups by chance, so the groups are fair to compare.
recruiting: The trial is currently looking for new people to take part.

S

secondary outcome: Extra results the trial measures alongside the main one.
side effect: An unwanted effect of a treatment, such as feeling sick, tired, or getting a rash.
single-blind: Either the participants or the researchers know who is getting the real treatment, but not both.
sponsor: The organisation that takes responsibility for the trial, such as a university, hospital, charity, or pharmaceutical company.

W

washout period: A break from previous treatments before starting the trial, so they don't affect the results.