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Animal Research
This entry reviews the five basic research designs available to investigators who study the behavior of nonhuman animals. Use of these experimental methods is considered historically, followed by a short review of the experimental method proper. Then, for each design, the discussion focuses on manipulation of the independent variable or variables, examples of testable hypotheses, sources of error and confounding, sources of variation within the design, and statistical analyses. The entry concludes with a section on choosing the appropriate research design. In addition, this entry addresses why it is important to choose a research design prior to collecting data, why certain designs are good for testing some hypotheses but not others, and how to choose a research design. This entry focuses on nonhuman animals, but the content generalizes directly to the study of behavior, either human or nonhuman.
The Experimental Method: Pathway to the Scientific Study of Nonhuman Animal Behavior
Through his dissertation written in 1898, Edward L. Thorndike initiated the controlled experimental analysis of nonhuman animal behavior. His use of the experimental method provided researchers interested in the evolution of intelligence, learning and memory, and mental continuity the opportunity to determine the causes of behavior. With the publication of his work in 1911 and the plea for objective methods of science by C. Lloyd Morgan, John B. Watson, and others, the use of anecdotal methods, so prevalent in that time, virtually came to an end. Thorndike's work helped establish psychology as first and foremost a science, and later a profession.
The experimental method requires at minimum two groups: the experimental group and the control group. Subjects (nonhuman animals) or participants (human animals) in the experimental group receive the treatment, and subjects or participants in the control group do not. All other variables are held constant or eliminated. When conducted correctly and carefully, the experimental method can determine cause-and-effect relationships. It is the only method that can.
Research Designs with One Factor
Completely Randomized Design
The completely randomized design is characterized by one independent variable in which subjects receive only one level of treatment. Subjects or participants are randomly drawn from a larger population, and then they are randomly assigned to one level of treatment. All other variables are held constant, counterbalanced, or eliminated. Typically, the restriction of equal numbers of subjects in each group is required. Independent variables in which subjects experience only one level are called between-subjects variables, and their use is widespread in the animal literature. Testable hypotheses include the following: What dosage of drug has the greatest effect on reducing seizures in rats? Which of five commercial diets for shrimp leads to the fastest growth? Does experience influence egg-laying sites in apple snails? Which of four methods of behavioral enrichment decreases abnormal behavior in captive chimpanzees the most?
The completely randomized design is chosen when carryover effects are of concern. Carryover effects are one form of sequence effects and result when the effect of one treatment level carries over into the next condition. For example, behavioral neuroscientists often lesion or ablate brain tissue to assess its role in behavioral systems including reproduction, sleep, emotion, learning, and memory. In these studies, carryover effects are almost guaranteed. Requiring all subjects to proceed through the control group first and then the experimental group is not an option. In cases in which subjects experience treatment levels in the same order, performance changes could result through practice or boredom or fatigue on the second or third or fourth time the animals experience the task. These so-called order effects comprise the second form of sequence effects and provide a confound wherein the experimenter does not know whether treatment effects or order effects caused the change in the dependent or response variable. Counterbalancing the order in which subjects receive the treatments can eliminate order effects, but in lesion studies, this is not possible. It is interesting that counterbalancing will not eliminate carryover effects. However, such effects are often, but not always, eliminated when the experimenter increases the time between conditions.
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