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  A Glossary of Experimental Design Terms

The purpose of this glossary is to define some of the fundamental concepts of research design with which you should be familiar before you sign up for your first experiment. The definitions may also be helpful for the part of the course dealing with research methods covered in January when we discuss these and other related concepts more thoroughly. Note that an introductory discussion of concepts related to research appears in Chapter 1 of the Gleitman text, pages 13 - 37. See also Gleitman's general glossary of terms, pages B1 - B 25.

 

Hypothesis:

...a proposition or an assumption that one attempts to verify (or refute) through experimentation or observation. An example of an hypothesis might be: "Students study more effectively in quiet than in noisy environments."

 

Experiment:

In an experiment, the experimenter deliberately manipulates one or more variables (factors) in order to determine the effect of this manipulation on another variable (or variables). An example might be measuring the effect of noise level on subjects' memorization performance of a list of standard nonsense syllables (such as ZUP, PID, WUX, etc.).

 

Independent Variable:

...the "treatment" variable that the experimenter hypothesizes "has an effect" on some other variable. (See Dependent Variable, below). In the example above, the independent variable would be the level of noise (in this case with three levels: low, medium, high). In an experiment, the independent variable is directly manipulated by the experimenter. But in an observational study, or when naturalistic observations are used, the independent variable is not directly manipulated by the experimenter, and the levels of the independent variable occur naturally and are already given when the study begins.

 

Experimental Group and Control Group:

In some experiments, the levels of the independent variable consist of only two: a treatment-present condition and, for comparison purposes, a treatment-absent, or no-treatment, condition. The group receiving the treatment-present condition (one of the two levels of the independent variable) is called the experimental group, and the group receiving the treatment-absent, or no-treatment, condition (the other level of the independent variable) is called the control group.

 

Dependent Variable:

...the variable that the experimenter hypothesizes is "affected by," or "related to," the independent variable. It is the "outcome" or "effect" variable, usually a measure of the subjects' performance resulting from changes in the independent variable. In the example, above, the dependent variable might be the number of nonsense syllables recalled correctly.

 

Error Variable:

In an experiment, the independent variable is likely not the only variable potentially responsible for observed changes in the dependent variable. Many other possible variables, such as the time of day, the mood of the subject, recent news events, or the weather, might also affect outcomes in an experiment. These error variables, often unnoticed, unknown, or unmeasured, may be responsible for observed variations in the dependent variable even within any one group in the experiment in which the level of the independent, or treatment, variable remains constant. Thus, error variables account for all the individual differences in responding not accounted for specifically by changes in the independent variable. Error variables must be controlled, for example by randomization.

 

Confounding Variable (or Confound):

A particular error variable whose possible effects on the dependent variable are completely consistent with the effects of the independent variable. The presence of a confounding variable precludes being able to ascribe the changes in the dependent variable exclusively to the independent variable. The changes could also be due to the confounding variable. Confounding variables must be controlled for, for example, through randomization or by holding them constant.

 

Control Variable:

One can prevent the effects of a specific, identifiable error variable from clouding the results of an experiment by holding this error variable constant. For example, if all subjects are the same age, then variations in age cannot act as an error variable. A variable that is thus held constant is called a control variable. (Of course one can then no longer generalize the results to those of ages other than the age selected for the experiment.)

 

Null Hypothesis and Alternative Hypothesis:

The null hypothesis refers to the statement that changes in the independent variable have no effect on the dependent variable and that therefore whatever difference was found between the experimental and control groups simply occurred by chance through the influence of random error variables. The alternative hypothesis refers to the statement that changes in the independent variable really have an effect on the dependent variable and that the difference in performance between the experimental and control groups was greater than what would be expected by chance through only the influence of random error variables. (In the example, above, the null hypothesis is that changes in noise level have no effect on subjects' recall scores. The alternative hypothesis is that changes in noise levels have an effect on subjects' recall scores.)

 

Debriefing:

In some experiments, it is not desirable that participants know the exact nature of the hypothesis being tested. (There is evidence in certain kinds of experiments that if participants know what the hypothesis is, they might, either consciously or unconsciously, respond in order to try to prove the hypothesis correct -or perhaps false! (rather than respond "naturally" and "honestly.") In such experiments the experimenter arranges to keep participants temporarily "in the dark" about the hypothesis until after the necessary data have been collected. Immediately thereafter, however, the experimenter is obligated to inform participants about the true nature of the hypothesis, why the experiment was designed as it was, and what previous investigators in the relevant areas had found. The experimenter might also ask participants to report to what extent they "saw through" the experiment and what they thought the experimenter was trying to find out. (If an experimenter has other participants yet to test in such an experiment, the experimenter might ask you to keep the information revealed in the debriefing session confidential until the conclusion of the whole experiment. Please cooperate with experimenters in this regard.)

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