Distinguish among internal, external, and construct validity

Going over Table 2.1 will help. However, be aware that students will not always understand these concepts the first time through. Fortunately, they will be re-exposed to these concepts many more times because these concepts are an organizing theme of the text. In the beginning, you might focus your discussion of construct validity on measures (using examples of invalid tests such as tests found in magazines and early intelligence tests). You will find that students easily understand external validity, especially if you use examples of studies that use rats as participants. In discussing internal validity, students will have a tendency to assume that a study has internal validity. You can prevent this by

  1. Telling students that the study has a flaw and their job is to find it and
  2. Introducing obvious confounds such as having the treatment group being composed of seniors and the comparison group being composed of sophomores.

A classic source of puzzles is

Huck, S. W., & Sandler, H. M. (1979). Rival Hypotheses: Alternative interpretations of data based conclusions. To get newer sources, you could have students bring in conclusions made in ads or news reports that have questionable validity.

Alternatively, you could have students read the following article:

Seechrest, L. & Walsh, M. (1997). Dogma or Data: Bragging Rights. American Psychologist, 32, 536-540.

Students will need some help going through the article (and you probably only want to cover selected sections of the first 3 1/2 pages), but it provides brief definitions of the three validities and makes two good points: :

  1. People should evaluate studies in terms of internal, construct, and external validity (the organizing theme of this text)
  2. Evaluating external validity is more than common sense. For example,
    1. Large sample size does not guarantee external validity.
    2. We should have specific reasons for doubting the generalizability of research. As the authors state, "Humans may doubt the generalizability to their own lives about the foraging strategies of rats, but they would not eat anything that causes a rat to drop dead."

 

One problem students will have is understanding that random assignment helps a study's internal validity whereas random sampling helps external validity. Scott Plous has devised a short student assignment that you can use to help students understand the difference between random sampling and random assignment.


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