Definition
Experimental group (also called the treatment group): the participants in an experiment or clinical trial who receive the intervention being tested, such as a new drug, device or therapy. Their outcomes are compared with a control group that receives a placebo, standard treatment or no intervention. In clinical trials, randomization decides who joins the experimental group.
- The experimental group receives the independent variable (the intervention); researchers measure its effect on the dependent variable (the outcome).
- Without a comparable control group, you cannot tell whether a change was caused by the treatment or by something else.
- A study can have more than one experimental group, for example two doses of the same drug.
Updated October 26, 2026
Learning how trials are designed? Trial design, randomization and treatment arms are part of the VIARES Clinical Research Associate course.
Contents
- What is an experimental group?
- Experimental group vs control group
- The experimental group and the scientific method
- How the experimental group is formed
- Study designs with experimental groups
- Examples of experimental groups
- How results are compared
- Challenges and ethical considerations
- Practitioner note
- FAQ
- Sources
- Where you learn this at VIARES
Every experiment asks one question: did the intervention make a difference? To answer it, experimental design uses two groups that are alike in every way except one variable. The group that receives the intervention is the experimental group.
What is an experimental group? Definition and meaning
An experimental group is the group of participants (or animals, plants or samples) in a study that is exposed to the variable under investigation. In science classes it is the group that gets the “change”; in medicine it is the group that gets the new treatment.
The same idea goes by several names:
- Treatment group: the most common synonym in medicine. ICH E10, the international guideline on choosing control groups, uses “test treatment” and “treatment group” language.
- Intervention group or experimental arm: used in trial protocols and registries.
- Treatment arm or study arm: each group in a trial is an “arm”; the experimental arm is the one receiving the investigational product.
The definition is the same in psychology, biology and clinical research. Only the intervention changes: a therapy technique, a fertilizer, or an investigational medicinal product.
Experimental group vs control group
The experimental group only has meaning next to a control group. The control group shows what happens without the intervention, so any extra effect in the experimental group can be attributed to the treatment.
| Aspect | Experimental group | Control group |
|---|---|---|
| Receives | The intervention being tested (new drug, dose, device, therapy) | Placebo, standard of care, an active comparator or no treatment |
| Role in the experiment | Exposed to the independent variable | Baseline for comparison |
| What it shows | Outcome with the intervention | Outcome without it (natural course, placebo effect, usual care) |
| Assignment | Usually by randomization | Same randomization, same eligibility criteria |
| Everything else | Same visits, assessments and follow-up | Same visits, assessments and follow-up |
ICH E10 and the FDA’s rules on adequate and well-controlled studies (21 CFR 314.126) describe five kinds of control: placebo, no treatment, dose-response (different doses), active treatment, and external or historical control. In each case, the experimental group is the one receiving the test treatment.
The experimental group and the scientific method
The experimental group is a key component of the scientific method. A researcher states a hypothesis (“drug X lowers blood pressure more than placebo”), and a null hypothesis (“there is no difference”). The intervention is the independent variable. The outcome, such as blood pressure at week 12, is the dependent variable.
If the two groups were comparable at the start and treated alike except for the intervention, a difference in outcomes can be attributed to the intervention. That is why experimental and control groups are essential for establishing causality. Observational studies, which have no assigned experimental group, can show associations but are more exposed to confounding variables.
How the experimental group is formed in a clinical trial
- Eligibility. Participants must meet the protocol’s inclusion criteria and none of the exclusion criteria.
- Informed consent. Each participant gives informed consent before any study procedure, knowing they may end up in either group.
- Random assignment. Randomization allocates each participant to the experimental or control group by chance. ICH E9 names randomization and blinding as the most important design techniques for avoiding bias.
- Allocation concealment. The person enrolling cannot know or influence the next assignment.
- Blinding. Where possible, participants and investigators do not know who is in the experimental group (blinding, as in a double-blind study).
Note the difference between random selection and random assignment. Random selection decides who enters a study from a population; random assignment decides which group each enrolled participant joins. Clinical trials rarely select participants at random, but they almost always assign them at random.
Study designs with experimental groups
| Design | Experimental group(s) | Typical use |
|---|---|---|
| Parallel two-arm trial | One experimental group vs one control group, followed at the same time | Most Phase III randomized controlled trials |
| Multi-arm or dose-response | Several experimental groups (different doses or drugs) and often a shared control | Phase II dose finding; platform trials |
| Factorial | Two interventions tested at once: A, B, A+B, neither | Answering two questions in one trial |
| Crossover | Each participant is in the experimental and the control condition, in random order | Stable chronic conditions; bioequivalence |
| Add-on | Experimental group gets standard therapy plus the new drug; control gets standard therapy plus placebo | When withholding treatment would be unethical |
Examples of experimental groups
Clinical trial example: the RECOVERY dexamethasone comparison
In the RECOVERY trial in hospitalized COVID-19 patients, 2,104 patients were randomly assigned to receive dexamethasone (the experimental group) and 4,321 to receive usual care alone (the control group). Within 28 days, 22.9% of the dexamethasone group and 25.7% of the usual care group died (age-adjusted rate ratio 0.83). Because assignment was random, the difference could be attributed to dexamethasone. The comparison was open-label: there was no placebo, which shows that a control group does not always need one.
Psychology example
A hypothetical study tests whether a 6-week mindfulness program reduces exam anxiety. Students are randomly assigned: the experimental group attends the program; the control group joins a waitlist and receives it later. Both groups complete the same anxiety questionnaire before and after.
Biology and science class example
In a controlled experiment with plants, the experimental group of seedlings receives a new fertilizer, and the control group does not. Light, soil, temperature and the amount of water stay the same, so only one variable changes and a difference in growth can be linked to the fertilizer.
How results are compared
At the end of a trial, statisticians compare the primary outcome between the experimental group and the control group, as planned in the protocol. The effect is reported as a difference, ratio or hazard ratio with a confidence interval and a p-value. The sample size is calculated in advance so that a real difference of the expected size has a good chance of being detected.
Most confirmatory trials analyze participants in the group they were randomized to, even if they stopped treatment. This intention-to-treat analysis keeps the benefit of randomization. The CONSORT 2010 statement asks trial reports to show how many participants were assigned to, received and were analyzed in each group.
Challenges and ethical considerations
- Selection bias: if investigators can choose who joins the experimental group, healthier patients may end up there. Randomization and allocation concealment prevent this.
- Imbalance by chance: in small trials, groups can still differ at baseline; stratified randomization helps.
- Dropouts and missing data: if more participants leave one group, the comparison weakens.
- Ethics: a control group may only receive placebo when that does not expose participants to serious harm; otherwise an active comparator or add-on design is used. Participants in the experimental group take on the unknown risks of a new treatment, which is why ethics review and informed consent are required.
Practitioner note
FAQ
What is the experimental group in an experiment?
The experimental group is the group that receives the treatment or change being tested. Its results are compared with a control group that does not receive it. If both groups are alike in every other way, a difference in results shows the effect of the treatment.
Is a treatment group the same as an experimental group?
Yes. In clinical research the experimental group is usually called the treatment group, intervention group or experimental arm. All of these terms describe the participants who receive the intervention under study.
What is the difference between the control group and the experimental group?
The experimental group receives the intervention; the control group receives a placebo, standard treatment or nothing. Both groups are selected by the same eligibility criteria and followed in the same way, so the control group shows what would have happened without the intervention.
Can a study have more than one experimental group?
Yes. Dose-finding trials often have several experimental groups that receive different doses, and multi-arm trials test several treatments against one shared control group. Factorial designs test two interventions alone and in combination.
What is an example of an experimental group in psychology?
In a study of a new therapy for exam anxiety, students randomly assigned to receive the therapy form the experimental group. Students on a waitlist, who receive it later, form the control group. Both complete the same anxiety measures at the same times.
Sources
- ICH. E10: Choice of Control Group and Related Issues in Clinical Trials.
- ICH. E9: Statistical Principles for Clinical Trials.
- ICH. E6(R3) Guideline for Good Clinical Practice (2025).
- U.S. eCFR. 21 CFR 314.126: Adequate and well-controlled studies.
- Schulz KF, Altman DG, Moher D. CONSORT 2010 Statement. BMJ. 2010.
- RECOVERY Collaborative Group. Dexamethasone in Hospitalized Patients with Covid-19. N Engl J Med. 2021.
- National Cancer Institute. NCI Dictionary: control group.
Where you learn this at VIARES
Section 6, “Clinical Trial Design & Clinical Research Roles and Responsibilities”, of the VIARES Clinical Research Associate course covers the study protocol, trial design and methodologies. In its assignment you explain stratified block randomization, double-dummy, crossover and factorial designs, why they are chosen and how they reduce bias. Section 10, “Investigational Medicinal Product Management”, covers comparators and placebo, the randomization process and un-blinding procedures. VIARES also offers the free Micro Certificates EXPERIMENTAL GROUP, CONTROL GROUP and RANDOMIZATION.
