Random Assignment in Research: Definition and Importance

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  • 0:07 Randomization
  • 2:01 Random Selection
  • 3:22 Random Assignment
  • 4:39 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.

In order to get the most accurate results, researchers must choose and assign their subjects in a random manner. In this lesson, we'll look at random assignment, random selection, and why they are important.


Charlene is a psychologist. She's interested in whether sitting on a jury will increase the level of patriotism that people feel. She goes to her local courthouse and gives jury members from several trials a short questionnaire that tells her how patriotic they feel. After they've served on a jury, Charlene gives them the same questionnaire and then compares their answers to see if they are feeling any more patriotic than before.

Now, there are a lot of things that can affect how patriotic a person feels. They could be hit with a high tax bill and feel less patriotic, or they could talk to a veteran and feel more patriotic. How does Charlene know that the results of her study are because people served on a jury and not because something else happened to influence her results?

The truth is that Charlene can't ever be 100% sure that her results are only the result of serving on a jury. However, one thing that she can do to increase the chances that they are is randomization, or the process of randomly selecting or assigning subjects.

Why does randomization work? Let's look at a specific example. Let's say that Charlene has evidence that tall people feel more patriotic than short people. When she goes to pick out her subjects, she notices a lot of tall people there. If she selects mostly tall people, will that make a difference to her results? If height is related to feelings of patriotism, then it might.

But what if Charlene didn't choose the tall people? What if she just randomly chose people of all different heights? In that case, she's more likely to get a good mix of people that represent the population at large. Because there's a mix of people, height is not as likely to affect her results. Let's look closer at two types of randomization: random assignment and random selection.

Random Selection

Imagine that Charlene goes to her courthouse, and there are 10 trials going on. She only needs to poll the juries from seven of the ten trials. How does she choose?

Charlene knows that three of the trials involve cases that will more than likely make people feel frustrated or disgusted with the government. As a result, they might not feel patriotic after sitting on those trials. So, Charlene decides to ignore those trials and just choose the jurors who are in the other seven trials.

Uh-oh. Charlene hasn't used randomization, and her results might be biased. Instead, she should have used random selection, or the process of randomly choosing which subjects to include in a study. It's easy to remember because of the word 'selection.' You are selecting which people to include in your study, and you are selecting them randomly. In Charlene's case, she should have flipped a coin or drawn names out of a hat in order to choose which trials to include.

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