Unit 4 Random Assignment�
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Lesson 1 What is Random Assignment?
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In the last unit you learned about the importance of considering sampling and selection in experiments.
Another thing consider is whether the researcher randomly assigned participants to conditions in an experiment.
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Lesson 1: What is Random Assignment?
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Let’s walk through some terminology so we can better understand random assignment.
Lesson 1: What is Random Assignment?
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When we say random assignment, we mean randomly assigning participants to groups.
For example, if you wanted to see the effect of water on plant growth you could take a group of plants and randomly decide which plants get watered once per week and which plants are watered once per month.
Lesson 1: What is Random Assignment?
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By participants, we mean the people or things that we are studying. A participant is a member of the sample that the researcher recruits from a population.
Population
Sample
Participant
Lesson 1: What is Random Assignment?
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When we talk about randomly assigning to groups or conditions, this refers to the different groups of participants that a researcher plans to compare to one another.
Conditions should only differ from each other based only on the experimenter’s question.
Let’s go through an example.
Lesson 1: What is Random Assignment?
Dr. Beans is interested in whether drinking coffee before an exam boosts test scores. In order to test this question she decides to give some students coffee, and other students water before an exam.
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Example
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Students in the coffee group are called the experimental group because the researcher thinks that coffee leads to better exam scores.
Students in the water group are called the control group because they are in a neutral condition (aka there is no reason to expect water to boost exam scores).
Example
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You might be thinking, if we don’t expect drinking water to do anything, then why do we have a control condition?
Example
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A control condition is important because it will allow Dr. Beans to compare the performance of the coffee group to a baseline.
Example
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Without the control group, how will Dr. Beans know if people who drink coffee perform better?
There must be a comparison.
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Example
Let’s check your knowledge! �
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Lesson 1: What is Random Assignment?
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Lesson 1: What is Random Assignment?
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Lesson 1: What is Random Assignment?
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2. Dr. M is interested in whether reading poetry leads to increased vocabulary compared to what students usually read in their free time, magazines. She has some students read poems every day for a week and others read magazines every day for a week. _______ is the experimental condition and _______ is the control condition.
Lesson 1: What is Random Assignment?
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2. Dr. M is interested in whether reading poetry leads to increased vocabulary compared to what students usually read in their free time, magazines. She has some students read poems every day for a week and others read magazines every day for a week. _______ is the experimental condition and _______ is the control condition.
Lesson 1: What is Random Assignment?
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Random assignment is important because it allows the researcher to determine that it was their manipulation that led to the results, and not other differences between the groups.
By making the groups relatively equal at the beginning, we can ensure that the results we see are from the experiment and not from some other variable.
Lesson 1: What is Random Assignment?
Let’s go back to Dr. Beans and consider the following scenario: �
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Lesson 1: What is Random Assignment?
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Dr. Beans had students sign up to participate in her study.�She put the first 50 students to sign up in the coffee group and the second 50 students to sign up in the water group.
These are her results.
�She found that students who drank
coffee earned better exam scores!
Example
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Do you think these findings are valid? Why or why not?
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We can’t trust these findings because the participants were not randomly assigned to groups. Another variable could be
causing this difference, for example, maybe the more motivated students signed up first and it is actually motivation, not coffee that is causing the group difference.
Example
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If Dr. Beans had randomly put students in conditions and found the same result, then that may cause us to believe that coffee boosts exam performance!
Example
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Are the following examples of random or non-random assignment?
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Are the following examples of random or non-random assignment?
Random
Random
Non-random
Non-random
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Lesson 2 Thinking About Random Assignment
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In the last lesson you read about the importance of random assignment.
Let’s go through another example.
Lesson 2: Thinking About Random Assignment
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Remember to use your critical thinking skills and think about the following:
1. What is the evidence?�2. Do the findings support the conclusions?�3. Does the sample allow the researcher to answer their question?
Lesson 2: Thinking About Random Assignment
Meditation Improves Student Sleep Quality
In 2015, a group of researchers studied whether meditating would lead to better sleep quality. At the beginning of the semester they gave students 5 hours of meditation training. At the end of the semester, the researchers realized they needed a comparison group, so they gave another group of students 5 hours of vocabulary lessons.
They found that students in the meditation group got, on average, 2.5 more hours of sleep per night than the students in the vocabulary group. The researchers suggest that meditation should be incorporated into school curricula as it helps students sleep better.
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Science News
Example
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Does the study convince you that meditation leads to better sleep? Why or why not?
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Do you think that the experimental design allowed the researchers to determine whether meditation leads to better sleep quality? Why or why not?
Meditation
Better Sleep Quality
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Do you think that the researchers used random assignment to conditions?
Why or why not?
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The researchers did NOT use random assignment.
Participants in one group were recruited at the beginning of the semester, and participants from another group were recruited at the end of the semester.
Before we think about why this is a problem for the researchers’ conclusions, let’s check your understanding:
Example
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What is the experimental condition in the study?
What about the control condition?
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You should have answered that the meditation group was the experimental condition, and the vocabulary group was the control or comparison condition.
Now, let’s discuss why this study cannot show that meditation causes better sleep.
Example
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The researchers didn’t randomly assign participants to conditions. This means that there could be lots of differences between the experimental and control group in addition to whether or not they meditated.
Therefore, we have no way of knowing whether the group differences in the number of hours slept are from meditating or from something else! Let’s go through some examples:
Example
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Example
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What could the researchers have done differently to show that meditating caused better sleep?
What could the researchers have done differently to show that meditating caused better sleep?
Answer: They should have collected the data from everyone at the same time and they should have randomly assigned people to the experimental or control group.
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Let’s Recap!
In this unit you learned about the importance of random assignment in experiments.
Random assignment is important because it ensures that an observed effect is caused by the researcher’s experimental manipulation and not some other variable.
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