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The most rigorous form of scientific investigation for testing hypotheses is the experiment. Experiments are based on a logical structure, or design, within which the investigator systematically introduces changes into natural phenomena and then observes the consequences of those changes. The purpose of an experiment is to support a cause-and-effect relationship between a particular action or condition (the independent variable) and an observed response (the dependent variable).

The essence of an experiment lies in the researcher's ability to manipulate and control variables and measurements, so that rival hypotheses are ruled out as possible explanations for the observed response. These rival hypotheses concern the potential influence of unrelated factors, called extraneous variables (also called nuisance variables or intervening variables). An extraneous variable is any factor that is not directly related to the purpose of the study, but that may affect the dependent variable. Extraneous variables can be extrinsic factors that emerge from the environment and the experimental situation or intrinsic factors that represent personal characteristics of the subjects of the study.

When extraneous variables are not controlled, they exert a confounding influence on the independent variable, that is, they contaminate the independent variable in such a way that their separate effects are obscured. For example, if we wanted to examine the effect of cryotherapy for relieving shoulder pain, and our subjects were on pain medication, the medication would be a confounding factor. If we observe a decrease in pain following treatment, we could not determine if the effect was due to the treatment, the medication, or some combination of the two. Other extraneous factors that could interfere with conclusions could be spontaneous healing or other treatments the patient is receiving. Experiments are designed to control for this type of confounding.

In reality, of course, clinical experiments seldom have the ability to completely eliminate confounding effects; however, even though causality can never be demonstrated with complete certainty, the experimental method provides the most convincing evidence of the effect one variable has on another. The purpose of this chapter is to examine issues of experimental control that must be addressed if the researcher is to have confidence in the validity of experimental outcomes.


To be considered a true experiment, a study must have three essential characteristics: The independent variable must be manipulated by the experimenter, the subjects must be randomly assigned to groups and a control or comparison group must be incorporated within the design.

Manipulation of Variables

Manipulation of variables refers to a deliberate operation performed by the experimenter, imposing a set of predetermined experimental conditions (the independent variable) on at least one group of subjects. The experimenter manipulates the levels of the independent variable by assigning subjects to varied conditions, usually administering the intervention to one group and withholding it from another. For example, ...

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