Chapter 2: Professor-Oriented Summary

Understanding Types of Validity

Students can grasp that internal validity deals with cause-effect relationships and that external validity deals with generalizing results. It is especially easy for them to understand that external validity is basically




However, they tend to have difficulty understanding construct validity.

One reason students have trouble with construct validity is that they don't really understand what constructs are. To help them understand constructs (and thus construct validity), emphasize that:

Students may confuse construct validity with external validity. To help them, tell them that external validity often refers to P(eople) and P(laces), whereas construct validity tends to focus on M(anipulations) and M(easures).

Critiquing Study's Validity

To get students willing and able to critique a study's validity, you might briefly go over table 2-1.

Students are pretty good at finding flaws with the external validity of a study (although they are fooled by large, non-random samples). In fact, they may be a bit overcritical of the external validity of research. Although individuals differ, we can still study individuals scientifically and find general rules. Science forms general laws about things that are unique all the time. Two examples:


Similarly, lab studies often do generalize to the real world.

Although students are skeptical of the external validity of research, they are a little too trusting of cause-effect claims. To motivate them to question these claims, you may want to stress the importance of knowing the cause for (1) finding cures and (2) being wise. 

To stress the importance of knowing the cause, you might

  1. Talk about people wrongly accusing parents of autistic children of parental neglect. The accusers blamed the parents based on a little dangerous knowledge--that  parents hold autistic children less than parents hold non-autistic children.
  2. Discuss facilitated communication.
  3. Talk about the problems resulting from jumping to the conclusion that high self-esteem is the cause of all kinds of good outcomes and that low self-esteem is the cause of all kinds of bad outcomes.
  4. Talk about all the unfortunate students who paid to have their papers typed, thinking that typing it would result in a better grade when a controlled study showed that teachers graded typed papers harder than hand-written ones.
  5. Recent research suggests that children's delinquent behavior is not a an effect of divorce. Divorced parents of delinquents may be feeling guilt ("I should have stuck it out for the sake of Johnny") that they shouldn't. 
To stress that not questioning the internal validity of statements can make one foolish (and conversely, questioning such claims can make one cleverer), you might mention the following):
  1. A person reportedly asked a reference librarian the following question: "Why were so many Civil War battles fought on national parks?"
  2. The funny case against bread
Criticizing the construct validity of a study may be best left until students have read Chapters 4 and 5. However, if you prepare a list of operational definitions and the construct labels assigned to those definitions, students will usually perceive a gap between the two.

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