How does science really know whether something is effective or not? Scientific inquiry answered this question long ago by trying to create specific controls in experiments. One such measure is the control group, or a group of similar people (animals, plants, etc) that are observed undergoing the same conditions as the test group, without receiving the thing the scientist wants to study. By creating as similar conditions as possible for a test and a control group, the scientist can determine the true effects of something he wants to test, by eliminating false results, especially if the only substantial variation is either receiving or not receiving the thing studied.
It’s easiest to think of the control group in terms of human studies, especially those that involve drug research. In a number of what are called double-blind studies, people participate without knowing whether they’re receiving a new drug. The other group gets a placebo and won’t know the results of the study until much later, if ever.
These clinical trials can reveal much about how effective a treatment might be, and they also show when it doesn’t work. For instance a control group not receiving the drug that reports improved symptoms in a much higher percentage than those reported by the group getting the drug would suggest the drug might not be as effective as hoped. Not only can potential effectiveness of medications be tested this way, but these studies can also show any side effects or long-term problems that might result.
Another way in which control groups can be formed is not quite as accurate and is called historical control. In this scenario the group is actually people who participated in an experiment in the past, selected as a way to compare results with a group that is currently being studied. Historical control is not always as accurate because theoretically a control group should be as greatly similar to the test group as possible.
In other words, a test can’t use samples that are as like, because the control group will be very different than the present group. Also, in controlled studies, part of the way that people try to get accurate results is by trying to make environment the same or similar for all participants. This can’t be done with a group studied in the past that forms the control, though a scientist may definitely look for previous control groups that have similarities to the people or other things currently being studied.
In all human groups, especially in lengthy experiments, it’s difficult to create control groups that are exactly similar to non-control groups, but scientists do have ways of narrowing down what they want. Those who participate in research may undergo lengthy physical examinations and fill out multiple questionnaires to make sure they are as similar to everyone else in the group being tested. Scientists might exclude participants who don’t show this similarity because additional factors they possess could skew results of testing, rendering the control group not as effective.