What are Controlled Experiments?

Determining Cause and Effect

A man and woman sit in chairs with electrical measurements on their heads, indicative of conditions in a controlled experiment.
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A controlled experiment is a highly focused way of collecting data and is especially useful for determining patterns of cause and effect. This type of experiment is used in a wide variety of fields, including medical, psychological, and sociological research.

Experimental Group And Control Group

To conduct a controlled experiment, two groups are needed: an experimental group and a control group. The experimental group is a group of individuals that are exposed to the factor being examined. The control group, on the other hand, is not exposed to the factor. It is imperative that all other external influences are held constant. That is, every other factor or influence in the situation needs to remain exactly the same between the experimental group and the control group. The only thing that is different between the two groups is the factor being researched.

Designing Controlled Experiments

In controlled experiments, researchers can use several different techniques in order to avoid introducing sources of bias into their studies.

  • Random assignment. In studies that use random assignment, research participants are randomly assigned to be in the experimental group or the controlled group. For example, imagine a study of a new drug in which all of the female participants were assigned to the experimental group and all of the male participants were assigned to the control group. In this case, the researchers couldn’t be sure if the study results were due to the drug being effective or due to gender. Random assignment is done in order to ensure that participants are not assigned to experimental groups in a way that could bias the study results. A study that compares an experimental and control group but does not randomly assign participants to the groups is referred to as a quasi-experimental study rather than a true experiment.
  • Blind or double-blind study. In a blind experiment, participants don’t know whether they are in the experimental or control group. For example, in a study of a new experimental drug, participants in the control group may be given a pill (known as a placebo) that has no active ingredients but looks just like the experimental drug. In a double blind study, neither the participants nor the experimenter knows which group the participant is in (instead, someone else on the research staff is responsible for keeping track of group assignments). Double-blind studies prevent the researcher from inadvertently introducing sources of bias into the data collected.


    If you were interested in studying whether or not violent television programming causes aggressive behavior in children, you could conduct a controlled experiment to investigate. In such a study, the dependent variable would be the children’s behavior, while the independent variable would be exposure to violent programming. To conduct the experiment, you would expose an experimental group of children to a movie containing a lot of violence, such as martial arts or gun fighting. The control group, on the other hand, would watch a movie that contained no violence.

    To test the aggressiveness of the children, you would take two measurements: one pre-test measurement made before the movies are shown, and one post-test measurement made after the movies are watched. Pre-test and post-test measurements should be taken of both the control group and the experimental group. You would then use statistical techniques to determine whether the experimental group showed a significantly greater increase in aggression, compared to participants in the control group.

    Studies of this sort have been done many times and they usually find that children who watch the violent movies are more aggressive afterward than those who watch a movie containing no violence.

    Strengths and Weaknesses

    Controlled experiments have both strengths and weaknesses. Among the strengths is the fact that results can establish causation. That is, they can determine cause and effect between variables. In the above example, one could conclude that being exposed to representations of violence causes an increase in aggressive behavior. This kind of experiment can also zero-in on a single independent variable, since all other factors in the experiment are held constant.

    On the downside, controlled experiments can be artificial. That is, they are done, for the most part, in a manufactured laboratory setting and therefore tend to eliminate many real-life effects. As a result, analysis of a controlled experiment must include judgments about how much the artificial setting has affected the results. Results from the example given might be different if, say, the children studied had a conversation about the violence they watched with a respected adult authority figure, like a parent or teacher, before their behavior was measured.