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Understanding Research - Preventing Bias

What is randomization?

Randomization is a process that assigns research participants by chance, rather than by choice, to either the investigational group or the control group. Each study participant has a fair and equal chance of being in the intervention group (for example, the one that gets the medication) or the control group (the one that gets the dummy pill).

The goal of randomization is to produce comparable groups in terms of participant characteristics, such as age or gender, and other key factors that affect the course that heart disease may 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.

How is bias in trials prevented?

Bias consists of human choices, beliefs, or any other factors besides those being studied that can affect a clinical trial's results.

Randomization is an important way of minimizing bias in a clinical trial. In a randomized trial, investigators use a computer program or a table of random numbers to assign each study participant to a group. 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.

What is a randomized, controlled, double-blinded trial?

The “gold standard” for research studies is the randomized, controlled, double-blinded trial.
Such studies use randomization to determine who gets put into which study group, for example, who takes the drug being tested and who takes a placebo pill. Controlled indicates that there is a control group not receiving the intervention being tested as a basis of comparison. Double-blinded means that neither the patient nor the physician knows which treatment they are getting and therefore to which group they have been assigned.

How do I know the results of a study can be trusted?

The size of a study is also important when determining how definitive the study’s results are likely to be. Researchers try to enroll as many participants as they need to get a statistically significant result — meaning a result that is not due to chance. The total number of participants needed to get such a result varies depending on what the trial’s researchers are hoping to uncover. In a small study, the participants may not be representative of all the people who have a specific problem being studied, such as high blood pressure or heart disease. When a study involves a larger number of participants, there is a better chance that those participants are representative. Again, though, it only matters that the number of participants leads to a statistically significant result.

As you read about research studies in the news, you can use this table to help you determine which studies have the most medically definitive findings.


Look For

Was the study prospective (investigators followed study participants forward in time) and were participants asked to undergo an intervention (a medication, activity, or treatment being tested)?

A study that is prospective and contains a specific intervention.

If the study concerns a new treatment or drug, what phase was the trial — Phase I, II, III, or IV?

At least a Phase III study.

Did the clinical trial have a control group — a group of participants that did not receive the intervention or instead received a placebo? Or were two treatments compared to one another?


A controlled study.

Were participants randomly assigned to either the investigational group or the control group?

A randomized study.

Did the researchers know which participants were in the interventional and control groups (single-blinded) or did neither the researchers nor the participants know what group the participants had been assigned to (double-blinded)?


A double-blinded study.

How many participants were enrolled in the study? Were the results statistically significant?

A study that is large enough for the results to be statistically significant -- not due to chance.

Next: Types & Phases of Clinical Trials


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