Reactivity of Experimental Arrangements & Assessment: Threats to External Validity

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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!

Reactive arrangements are when subjects change their behavior because they are participating in an experiment. In other words, they don't act the way they normally would. For example, are Mrs. Prim's students working really hard on fractions because that's what they would normally do or because they know they are participating in a study? If it's because they're in a study, then they are reacting to the arrangements of the study, and the results might not be true in the real world.

Another scenario where reactive arrangements come into play is when behaviors are being observed. For example, if Robin decides to do a study looking at how nice people are to strangers, she might run into reactive arrangements because people are likely to be nicer to other people, including strangers, if they know they are being observed.

You might be thinking that psychologists could get around reactive arrangements by just not telling the subjects that they are in an experiment. After all, if they aren't aware that they are part of a study, they won't change their behavior. The problem with this is that it violates the ethics of research. All research studies, including psychological ones, require informed consent of the participants; that is, before being part of the study, the subjects have to agree to the terms of the study.

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