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Reactivity of Experimental Arrangements & Assessment: Threats to External Validity Video

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  • 0:08 External Validity
  • 1:27 Reactive Arrangements
  • 3:02 Reactivity of Testing
  • 4:17 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 say something about what happens in the real world. But what happens if subjects react to the experimental conditions? In this lesson, we'll look at two reactivity threats to external validity.

External Validity

Robin is a psychologist, and she's interested in studying whether a math intervention helps students work better with fractions. She believes that the intervention will make students more adept at all aspects of working with fractions. So, she enlists the help of a teacher at a local school. To start with, Robin will give the students a fraction pretest so that she can know how much they know about fractions at the beginning.

Then the teacher, Mrs. Prim, will implement the intervention in her class. After six weeks with the intervention, Robin will give the kids a posttest to see how much more they know about fractions. If Robin's hypothesis is right, the students will be a lot better at fractions after the intervention than they were before.

But even if the students do a lot better on the posttest than they did on the pretest, does that mean that the intervention is perfect and should be used in every single classroom in the country? Maybe not. External validity is the extent to which results of a study can be generalized to the world at large. But there are some threats to external validity that researchers often face. One of them is reactivity, which is any reaction by the subjects to some aspect of the experiment. Let's look at two examples of reactivity: reactive arrangements and testing reactivity.

Reactive Arrangements

So, Robin and Mrs. Prim give the students a fraction intervention to see if it will help them learn fractions better. But the students know that they are part of a study, so they work extra hard on fractions. After all, they want to make their teacher proud!

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