Randomization is a process that assigns research participants by chance, rather than by choice, to either the investigational group or the control group of all phase III (and some phase II) clinical trials. (See What Is a Clinical Trial? for information about the different phases and types of clinical trials.)
Each study participant has a fair and equal chance of receiving either the new intervention being studied (by being placed in the investigational group), or of receiving the existing or "control" intervention (by being placed in the control group).
The goal of randomization is to produce comparable groups in terms of general participant characteristics, such as age or gender, and other key factors that affect the probable course the disease would take. In this way, the two groups are as similar as possible at the start of the study. At the end of the study, if one group has a better outcome than the other, the investigators will be able to conclude with some confidence that one intervention is better than the other.
A randomized, controlled trial is considered the most reliable and impartial method of determining what medical interventions work the best. (For more information about the importance of these factors, see Which Study Results Are the Most Helpful in Making Cancer Care Decisions?)
How Does Randomization Prevent Bias?
Research participants are randomized in clinical trials so that bias does not weaken the study results. Bias consists of human choices, beliefs or any other factors besides those being studied that can affect a clinical trial's results. If physicians or participants themselves choose the group, assignments might be personally influenced and therefore unevenly slanted toward one side or the other.
For instance, if a study is not randomized, physicians might unconsciously assign participants with a more hopeful prognosis to the experimental group, thus making the new therapy seem more effective than it really is. Conversely, participants with a less hopeful prognosis might pick the experimental treatment, leading it to look less effective than it really is.
Randomization prevents such bias. In a randomized trial, investigators use a computer program or a table of random numbers to assign each study participant to a group.
To further eliminate bias, randomized trials are sometimes "blinded" (also called masked).
Single-blinded trials are those in which participants do not know which group they are in - and therefore which intervention they are receiving - until the conclusion of the study.
Double-blinded trials are those in which neither the participant nor the investigators know to which group the participant has been assigned until the conclusion of the study.
Blinded studies are commonly used in cancer prevention clinical trials but are rarely used in trials investigating new treatments.
How Do Control and Investigational Groups Differ?
Informed consent documents for each trial give specific details about that trial's design and the interventions involved for both the control and investigational groups. (See Digest Page: Protecting Participants in Clinical Trials for more about informed consent.) In general, however, the difference between control and investigational groups can be described as follows, depending on the type of trial.
Cancer treatment trials test the safety and effectiveness of new drugs, biological agents, techniques, or other interventions in people who have been diagnosed with cancer.
The control group receives the most widely accepted treatment (standard treatment) for their cancer. Placebos are very rarely used in treatment trials.
The investigational group receives the new agent or therapy being tested.
Cancer prevention trials evaluate the safety and effectiveness of ways to reduce the risk of developing cancer in people who do not have cancer. Action studies focus on participants doing something, such as quitting smoking, that may prevent cancer. Agent studies (also called chemoprevention studies) focus on participants taking something, such as medicines, vitamins or food supplements, that may prevent cancer.
The control group receives the most widely accepted prevention intervention for a type of cancer. If there is not an existing accepted prevention measure, "usual care" or a placebo is taken.
The investigational group takes the agent or conducts the action that is being studied.
Cancer screening and early detection trials assess the effectiveness of new or possibly improved means of detecting the earliest stages of cancer in people who have no symptoms of cancer.
Cancer diagnosis trials develop better tools for doctors to use in classifying types and phases of cancer, and in managing the care of people with cancer.
Cancer quality of life/supportive care trials test interventions designed to improve quality of life for people with cancer and their families.
The control group (either cancer patients or their caregivers) receives standard treatment or usual care, whether it is a therapy or psychosocial intervention for nutrition problems, infection, pain, nausea, sleep disorders, depression, or other effects of cancer or its treatment.
The investigational group (either cancer patients or their caregivers) receives the new intervention under study.
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