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Participant Variables that Affect Internal Validity

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  • 0:06 Internal Validity
  • 1:23 Self-Selection Bias
  • 2:17 Demand Characteristics
  • 3:01 Good-Subject Bias
  • 3:53 Lesson Summary
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Lesson Transcript
Instructor: Natalie Boyd

Natalie is a teacher and holds an MA in English Education and is in progress on her PhD in psychology.

Sometimes, participants in an experiment can change the outcome. In this lesson, we'll look at some of the extraneous variables caused by participants: self-selection bias, demand characteristics, and good-subject bias.

Internal Validity

Ollie is doing a study on the effects of light on factory workers. Specifically, he wants to know if brighter lights will result in more productive work. He goes to a factory that makes computer parts and asks for volunteers for a study. After he gets his volunteers, he puts them into a room of their own and asks them to make computer parts as usual. Each week, he totals up the number of computer parts they make. Also each week, he adjusts the level of light they have in the room. If what he is thinking is right, then the weeks where the lights are brightest will produce the most computer parts.

Internal validity is the extent to which a study proves that only the independent variable is causing the changes in the dependent variable. In Ollie's case, he wants to prove that the light level (the independent variable) is causing changes in the productivity of the workers (the dependent variable). But wait. What if the brighter lights make the room warmer? Because they feel hotter, the subjects might not do as much work. This is an example of an extraneous variable, or a factor besides the independent variable that affects the dependent variable. There are several types of extraneous factors. Let's look at a few that involve the participants.

Self-Selection Bias

Remember that Ollie asked for volunteers from among the workers at the factory. Psychological studies are done on volunteers, which poses a threat to internal validity known as self-selection bias. Think about this: what types of workers at the factory are most likely to volunteer for Ollie's study? Probably workers who are enthusiastic. They are most likely the same types of people who will volunteer to take on additional responsibility. They might be better at their jobs than non-volunteers, who just don't care one way or another.

Because they are enthusiastic go-getters, the subjects themselves might be the type of people who are always trying to improve and be more productive. As a result, Ollie won't know whether the results are because of his manipulation of the lights or because the self-selected subjects are the type of people who are regularly trying to be more productive.

Demand Characteristics

Another participant-influenced extraneous variable is known as demand characteristics, or the Hawthorne effect. This happens when subjects figure out what the researcher is studying and change their behavior accordingly. For example, what if the workers in Ollie's study notice that the only thing changing from week to week is the level of light in the room where they're working? They might suspect that Ollie is studying the effect of light on productivity.

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