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An experiment is a highly controlled research scenario. It entails the intentional manipulation of one variable (the independent) in order to assess its causal impact on another variable (the dependent or outcome variable). The experiment is considered the best research design for examining cause and effect relationships. The strength of the experiment is found in its ability to control key study conditions -i.e. the deliberate creation of experimental and control groups, the intentional manipulation of the independent variable(s) and the careful measurement of the dependent variable.
The experimental group is exposed or subjected to the ”experimental condition” – i.e. the independent variable being investigated for its alleged causal impact. The control group is not exposed to the independent variable and thus serves as a base for critical comparison in assessing any causal impact. The experimental and control groups are configured to be virtually identical – i.e. they should resemble each other in all significant ways. This condition is best achieved via a random assignment process. Random assignment requires that chance and chance alone determines who is assigned to each group. For example, the names of all volunteers for an experiment might be placed in a hat. The first name drawn from the hat might be assigned to the experimental group, the second to the control group, and so on.
Once the experimental and control groups are established, the intentional manipulation of the independent variable takes place. After the independent variable has been introduced into the experimental group, the dependent variable is then measured in both groups. If the observed outcome on the dependent variable is not the same in the two groups, the difference can be attributed to the independent variable, the only condition that differs between the two groups. Consider the following example: A researcher wants to see if there is a causal connection between chewing nicotine gum and a reduction in smoking. Smokers are randomly assigned to the experimental and the control groups. The experimental group is given nicotine gum to chew; the control group does not receive the nicotine gum. At the end of the study period, smoking activity is measured in both groups. If members of the experimental group are smoking less than members of the control group, the reduction is attributed to the nicotine gum.
The experimental design is superior to other research designs with regard to the issue of internal validity – i.e. the ability to correctly assess the causal connections between variables. The experiment is particularly strong in its ability to control or eliminate many known threats to internal validity -i.e. conditions that undermine our ability to say if one variable causally impacts another. For instance, events that coincide with the timing of a non-experimental study might confound the study’s causal analysis. Consider for instance a simple before/after study trying to assess the impact of a driver’s ed program on students’ driving practices. The study’s causal analysis would be compromised if a celebrity were involved in a serious car accident during the study period. This is known as a history threat to internal validity. The experiment, with its use of both an experimental and a control group, is able to ”eliminate” the history threat when assessing any causal connections. History occurs to both groups and thus becomes an irrelevant factor in any causal assessment. There is a downside to the experiment – its contrived nature makes it weak on external validity-the ability to accurately generalize findings from experimental to nonexperimental conditions.
- Frankfort-Nachmias, C. & Nachmias, D. (2000) Research Methods in the Social Sciences. Worth, New York.
- Ruane, J. (2005) Essentials of Research Methods: A Guide to Social Science Research. Blackwell, Malden, MA.