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Carryover Effects & How They Can Be Controlled Through Counterbalancing

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  • 0:05 Within-Subject Design
  • 1:30 Carryover Effects
  • 2:58 Counterbalancing
  • 4:35 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.

Within-subjects research has a lot of advantages, but one disadvantage is the possibility of carryover effects. In this lesson, we'll examine carryover effects and how they can be controlled through a counterbalanced design.

Within-Subjects Design

Julissa is a psychologist who is interested in studying whether listening to music while working will help students write better. She decides to put the students into a room with music playing and have them write a short essay in response to a prompt.

Afterward, she'll put them into a room with no music and have them write again. Then, she'll compare their two essays to see which one is better. If the essays in the quiet room are better, then Julissa will know that listening to music does not help students write essays better.

Julissa is utilizing a within-subjects design for her study, which is when each subject is given every possible treatment, and his or her results are compared across the treatments to see which is best. In Julissa's case, her subjects will be given two different treatments (music and no music), and then Julissa will see which of the treatments works better.

When you think about a within-subjects design, think about all of the possible scenarios held within a single person. That's essentially what's going on: the researcher is trying out different scenarios on a single set of people, over and over, so that they can see which one works better.

Though there are many advantages to a within-subjects design, there are also some disadvantages. Let's look closer at one disadvantage, carryover effects, and how to combat it with counterbalancing.

Carryover Effects

OK, so Julissa puts her subjects into a room with music playing and gives them a prompt. They write their essay. Then she moves them to a quiet room and gives them another prompt, and they write another essay on a different prompt that has been shown to be equivalent to the first.

But wait a minute: what if the second essay is better because they were 'warmed up' from writing the first essay? How does Julissa know that her subjects are writing better because the room is quiet or because they have already written an essay on that day?

Julissa's uncovered carryover effects, which happen when one treatment affects subsequent treatments. In Julissa's case, her subjects are getting to practice essay writing in the music-filled room, and that practice is making them better in the quiet room.

You can remember carryover effects because the effects of one treatment are carrying over to the next treatment. If a subject learns a new skill or practices a skill, or has some other encounter in the first condition that then makes them better at the next condition, carryover effects are in play.

Why is this a bad thing? Carryover effects can make it difficult to discern how well a treatment works. Perhaps under normal circumstances people actually write better when music is playing in the background.

But because Julissa had the subjects write in the music-filled room first, they are able to practice, and her results show that they did better in the quiet room. It's not the room that made a difference, but the order of the treatments.

Counterbalancing

So how can Julissa get rid of the carryover effects? There's a simple and elegant solution to carryover effects. Think about it like this: Julissa needs to know if her results are because the quiet room is better or because they got to practice writing in the music-filled room. So all she has to do is make sure that half of her subjects get the music condition first and half of them get the quiet condition first.

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