8.6: Summary of Experimental Research

Key Takeaways

• An experiment is a type of empirical study that features the manipulation of an independent variable, the measurement of a dependent variable, and control of extraneous variables.
• An extraneous variable is any variable other than the independent and dependent variables. A confound is an extraneous variable that varies systematically with the independent variable.
• Experimental research on the effectiveness of a treatment requires both a treatment condition and a control condition, which can be a no-treatment control condition, a placebo control condition, or a wait-list control condition. Experimental treatments can also be compared with the best available alternative.
• Experiments can be conducted using either between groups, matched groups, or repeated measures design. Deciding which to use in a particular situation requires careful consideration of the pros and cons of each approach.
• Random assignment is a fundamental element of experimental research. The purpose of these techniques is to control extraneous variables so that they do not become confounding variables.
• Studies are high in internal validity to the extent that the way they are conducted supports the conclusion that the independent variable caused any observed differences in the dependent variable. Experiments are generally high in internal validity because there is initial equivalence of the participants in each IV group, and there's ongoing equivalence of the IV groups.
• Initial equivalence and ongoing equivalence provides internal validity because they show that nothing other than the IV could have caused changes in the DV. Initial equivalence is usually created through random assignment. Ongoing equivalence is usually created through having initial equivalence, and then attempting to control all other extraneous variables.
• It is important to standardize experimental procedures to minimize extraneous variables, including experimenter expectancy effects.
• It is important to conduct one or more small-scale pilot tests of an experiment to be sure that the procedure works as planned.

What's Next?

Now that we've covered the components of experiments, we're going to talk about other types of studies.

Exercises

• Practice: List five potential IVs that can be randomly assigned and manipulated by the researcher in an experiment. List five potential IVs that cannot be randomly assigned and manipulated by the researcher in an experiment.
• Practice: For each of the following topics, decide whether that topic could be studied using an experimental research design that included randomm assignment to reach initial equivalence between the IV conditions, and explain why or why not.
  • Effect of parietal lobe damage on people’s ability to do basic arithmetic.
  • Effect of being clinically depressed on the number of close friendships people have.
  • Effect of group training on the social skills of widowed men.
  • Effect of paying people to lose weight.
• Discussion: Imagine that a study found that writing a story to learn 10 vocabulary words improved memory more than re-writing the same 10 words and their definitions multiple times by the same participants. If the story condition was always tested after the re-writing condition, explain a fundamental problem with this research design and at least two ways that it might be corrected.
• Discussion: For each of the following topics, decide which would be better.
  • You want to test the relative effectiveness of two training programs for running a marathon.
  • Using photographs of people as stimuli, you want to see if smiling people are perceived as more intelligent than people who are not smiling.
  • In a field experiment, you want to see if the way a donation letter is written affects whether or not passersby give him any money.
  • You want to see if concrete nouns (e.g., dog) are recalled better than abstract nouns (e.g., truth).
• Discussion: Imagine a study in which you will visually present participants with a list of 20 words, one at a time, wait for a short time, and then ask them to recall as many of the words as they can. In the stressed condition, they are told that they might also be chosen to give a short speech in front of a small audience. In the unstressed condition, the participants are not told that they will not have to interact with anyone else. What are several specific things that you could do to standardize the procedure?