Threats to External Validity: Sample Characteristics, Stimulus Characteristics & Experimental Arrangements

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  • 0:06 External Validity
  • 1:50 Sample Characteristics
  • 3:06 Stimulus Characteristics
  • 4:17 Multi-treatment Interference
  • 5:33 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.

The goal of research is to generalize the results, but there are several things that can get in the way. This lesson examines three threats to external validity: sample characteristics, stimulus characteristics, and multiple-treatment interference.

External Validity

Fran is a psychologist who wants to study innovative thinking and how people use it to solve problems in the real world. Because she wants to be able to figure out how to stimulate innovative thinking, she comes up with an experiment to do in a lab.

Fran wants to know if reading a short list of problem-solving strategies will help more than reading an article about an innovative thinker. So, she gathers a bunch of participants and gives them each a list of problem-solving strategies, has them read the list and then has them solve a puzzle. She times them to see how fast they are at solving the puzzle.

Then, Fran gives them an article about a famous innovative thinker, like Steve Jobs. After they've read the article, they have to solve a similar puzzle. Fran times them on that one, too, to see if their time is faster after having read the article.

Fran believes that the faster the person solves the puzzle, the stronger they are at problem solving. As a result, she should be able to tell whether the list or the article is better at stimulating problem solving depending on how fast subjects are at solving the puzzle after they've been exposed to one of the treatments.

Of course, Fran isn't interested in just what people do in a lab. She's really interested in how things work in the real world. External validity is the extent to which the results of an experiment can be generalized to the world at large.

There are several problems that psychologists, like Fran, run into when planning experiments. Let's look at three threats to external validity: sample characteristics, stimulus characteristics and experimental arrangements.

Sample Characteristics

So, Fran is testing how quickly people solve puzzles as an indicator of how much innovative and problem-solving thinking they are doing. But what if the people she picks are just better at solving puzzles than others?

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