Random Assignment in Psychology: Definition, Example & Methods

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  • 0:05 What is Random Assignment?
  • 1:14 Experimental Design
  • 2:10 Experimental and…
  • 3:22 Making Random…
  • 4:29 Lesson Summary
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Lesson Transcript
Instructor: Andrea McKay

Andrea teaches high school AP Psychology and Online Economics and has a Masters degree in Curriculum and Instruction.

Random assignment is used by research psychologists studying human behavior. In this lesson, you'll discover why random assignment is crucial to methods of experimentation and research design.

What Is Random Assignment?

Psychologists use experiments to investigate how manipulation of one factor causes a change in another factor. Scientists refer to these factors as one of two kinds of variables. The independent variable is that first factor: the one whose influence we're trying to measure. An independent variable doesn't change based on the other variables. The second factor - the one being influenced by changes - is called a dependent variable. This kind of variable changes based on the independent variable. Experiments are the best way to determine cause and effect relationships between these variables.

Psychologists rely on random assignment to assign subjects to different groups in an experiment. Random assignment leaves it completely up to chance to determine which subjects receive the critical part of the experiment, which is imperative for determining that the independent variable is indeed what creates the result. Randomly assigning subjects helps to eliminate confounding variables, or variables other than the independent variable that could cause a change in the dependent variable.

Experimental Design

Suppose one day while studying for a test, you notice that you seem more focused and productive while you are listening to music. In fact, you think it's possible that listening to music while studying helps you earn better grades on tests. You have been taking psychology courses, and armed with the love of science, you decide to conduct an experiment to see if your hypothesis is correct.

You decide to test your hypothesis on the 300 students in your college introduction to psychology class. What is the independent variable in your experiment?

Remember that the independent variable is the part of the study that is manipulated or changed to determine a result. In your experiment, you will manipulate whether or not students listen to music while studying, so listening to music is the independent variable. The dependent variable then will be the subjects' scores on the test. The dependent variable shows the effect of the manipulation.

Experimental and Control Groups

To test the independent variable, you will need an experimental group and a control group. The experimental group is the group who receives the critical part of the experiment, the treatment. This is the group who will listen to music while studying.

But to know if the music has an effect on test scores, you will also need to compare the results of the experimental group to a control group, a group which doesn't receive the critical part of the experiment. In this case, our control group won't listen to music while studying.

How will you decide who is in the experimental group and who is in the control group? What about allowing students to choose which group they're in? No, that won't work. Maybe the students who choose to listen to music are already better students who excel at focused studying. We can't assume the results will be valid.

Okay, so how about picking the experimental group based on a first-come basis? Sorry, choosing to put the first 150 students who come to class in the experimental group is also not random assignment. Maybe those students who get up earlier to make it to class on time typically perform higher on tests because they get more sleep. Random assignment is the only way to eliminate other variables that could influence your results.

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