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Experimental Group: Definition, Lesson & Quiz

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Lisa Roundy

Lisa has taught at all levels from kindergarten to college and has a master's degree in human relations.

An experimental group is used in an experiment to test a variable. It is the group to which the test variable is administered. Learn more about how an experimental group works in this lesson, and test your knowledge with a quiz at the end.

We also recommend watching Two-Group Experimental Designs: Definition & Examples and What is a Control Group? - Definition and Use in Research

Definition of an Experimental Group

You usually store your microwave popcorn in the kitchen cabinet, but you read an article that recommends storing it the refrigerator. You want to find out if microwave popcorn actually pops better if it is stored in the refrigerator. You have two bags of popcorn so you keep one in the cabinet and place the other in the refrigerator. The next day you pop each bag of popcorn for the same amount of time in the microwave. You measure the amount of popcorn that popped in each bag to see which bag produced more.

You just created an experiment to test whether storing the popcorn in the refrigerator makes it pop better. In this example, your experimental group is the bag of popcorn you placed in the refrigerator. An experimental group is the group that receives the variable being tested in an experiment.

Experimental Groups in Action

There must be at least two groups in any valid experiment: The experimental group and the control group. The control group is the group in an experiment that does not receive the variable you are testing. For the experiment described above, the bag of popcorn that remained stored in the cabinet is the control group. The only difference in the two groups is where the popcorn is stored. This means that storing the popcorn in the refrigerator is the variable. A variable is the condition that is allowed to change.

There are two important things you need to know when you are setting up an experimental group. First you must know exactly what variable you are going to be testing. Then you must know how you are going to measure the results from the experimental group. In order to measure the results you must have some way of making a comparison. Comparing the results from the experimental group with the results of the control group is one way to measure the results you obtain from the experimental group.

Think about how the results of the popcorn experiment were measured. You measured the amount of popcorn that popped in each bag to see which bag produced more. Let's say that the control group produced 4 cups of popcorn and the experimental group produced 5 cups of popcorn. What conclusion could you make from this information?

In order for you to know exactly what causes a difference in the results between groups, only one variable can be measured at a time. For this reason it is important that you only test one variable in the experimental group. If you tried to test more than one variable, you would not be able to tell which one was responsible for any differences that occurred.

For example, if you popped each bag of popcorn in the previous experiment at different power settings on the microwave you would have two variables in the experimental group. You would not know if the different results in the two groups was due to storing the popcorn at different temperatures or using different power settings on the microwave.

Let's look at the following experiments and identify the experimental group in each one.

  • An experiment is testing if adding a fertilizer to the soil will make plants grow larger. One group of plants has no fertilizer added to the soil. The other group is grown in soil with added fertilizer. The experimental group in this example is the one with the fertilizer added to the soil.
  • An experiment is testing the effectiveness of a new medication for depression. One group is given the new medication. The other group is given a pill that contains no medication. The experimental group in this example is the one that is given the new medication.
  • An experiment is testing whether salt makes water boil faster. There are two pots heated at the same temperature and containing the same amount of water. Salt is added to only one pot. The experimental group is the pot with the salt added to it.

Lesson Summary

An experimental group is the group in an experiment that receives the variable being tested. One variable is tested at a time. The experimental group is compared to a control group, which does not receive the test variable. In this way, experimental groups are used to find answers in an experiment.

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