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As useful as other methods are for collecting data, conducting experiments occupies a central place in our repertoire of research methods. One reason is that experiments allow us to do something other methods can’t—they can identify the causes of behavior. With other methods, we can speculate about the causes, but we do not have enough control over the situation to make a firm determination. However, when a researcher designs an experiment, the goal is to control as many aspects of the experimental situation as possible to draw conclusions about the causes of the outcome.

The following are the essential features you will find in an experimental research design:

 The experimental group consists of the participants who receive the special treatment of interest to the researcher.

 The control group does not receive the special treatment and provides a baseline against which the experimental group can be compared.

 The participants are randomly assigned to either the experimental or the control group. Because this assignment is made by chance, the two groups will likely start out being very similar to each other, without any systematic differences that could affect the outcome of the experiment. Often researchers will assess the participants on the outcome variable before the experiment begins to make sure that the two groups are the same at the start of the experiment.

 The independent variable is the special treatment the researcher hypothesizes will make a difference between the experimental and control groups after the experiment. The assumption is that the independent variable is the cause of any change we observe following the experiment.

 The dependent variable is the outcome of interest to the researcher. We measure it at the end of the experiment to see whether manipulating the independent variable has produced the expected effect.

Experimental research design: A research design in which an experimental group is administered a treatment and the outcome is compared with a control group that does not receive the treatment.

Experimental group: The group in an experiment that gets the special treatment that is of interest to the researcher.

Control group: The group in an experiment that does not get the special treatment and provides a baseline against which the experimental group can be compared.

Independent variable: The variable in an experiment that the researcher manipulates.

Dependent variable: The outcome of interest to the researcher that is measured at the end of an experiment.

If we look at an example of experimental research, illustrated in Table 2.6 on the next page, this terminology will have more meaning for you. A study looked at the impact of attending Head Start on cognitive and behavioral development (Zhai, Brooks-Gunn, & Waldfogel, 2014). All children were eligible to enroll in Head Start, but there were not enough spaces for all of the applicants. The children who were randomly chosen to participate in the Head Start program became the experimental group and the other children who could not be enrolled became the control group. In this study, the outcome measure, or dependent variable, was the children’s cognitive and social-behavioral development. By the end of first grade, the researchers found that the children who had attended Head Start had better cognitive scores and parent-reported behavioral development than the children who did not attend Head Start. Because the two groups of children were similar at the start of the experiment, and because the only relevant difference between the groups during the intervention was whether the children attended Head Start or not, the researchers could conclude that participating in Head Start was the cause of the difference they observed at the end of the experiment.

Table 2.6

This table shows how experimental design (in this case an experiment to study the effect of attending Head Start) is carried out, starting with a sample of the population of interest to the researcher and ending with results that can be interpreted.

Active Learning: Experimental Research Design gives you an opportunity to review the terminology used in experiments and to check that you can recognize each element when you see it in the description of an experiment.

Active Learning: Experimental Research Design

You can test your understanding of experimental research design by identifying the components of this research designed by Beth Hennessey (2007) to measure the effects of a program to build social competence in a sample of school-age children. A total of 154 fourth graders in eight classrooms participated in the study. In half the classrooms, teachers used the Open Circle Program, a social skills training program that “encourages students, teachers and administrators to learn and practice communication, self-control and social problem-solving skills” (Hennessey, 2007, p. 349). The classroom teachers rated their students’ social competence in the fall before the program began and again in spring after the program ended. Based on the teachers’ reports, Hennessey concluded that the students in the classrooms that used the Open Circle Program training showed greater improvement in their social skills and problem-solving behavior than students who didn’t receive this training.

 From the description of this experiment, identify the following:

 Experimental group: ____________________________________________

 Control group: __________________________________________________

 Independent variable: _________________________________________

 Dependent variable: ___________________________________________

Answers:

Experimental group: The group that received the Open Circle Program training

Control group: The group that did not receive the Open Circle Program training

Independent variable: Whether the group received the social skills training or not

Dependent variable: The measure of social competence and problem solving

It may have occurred to you by now that, as appealing as the experimental method might be, it cannot answer many of the questions that are of great interest to us as developmentalists. There are many situations we could never ethically create as experiments. For example, if we want to study the effects of divorce on children, we cannot randomly assign half our sample of families to divorce while the other half stays married. Other research designs are necessary to pursue answers for these types of questions.