Scientists always need something to compare to in order to make sure that the independent variable is causing the changes to the dependent variable in their experiment.  This is especially important when living things are involved in an experiment, as living things are too unpredictable and complex to make constant.

In order to do this, a control group is used.

A control group is a standard for comparison used to make sure the changes in the dependent variable are caused by changes in the independent variable and not other factors.

1. Gather a large group of test subjects

- keep as many things about them the same as possible
 

color	health	breed/type	background
age	sex	diet	environment

2. Divide the test subjects into an experimental group and a control group
 

Experimental Group - actually does the experiment

Control Group - does everything the same EXCEPT for the independent variable

3. Do the experiment with the experimental group
4. Compare the results to the control group
5. Make a conclusion

Example:

Problem:

Determine the relationship between the number of roses on a rose bush and the amount of fertilizer used.

Independent Variable = the amount of fertilizer used

Dependent Variable = the number of roses on a rose bush

Constants = color of the rose, amount of water given, amount of sunlight, type of soil

(EVERYTHING should be constant except the independent and dependent variables!)

Describe the experimental group = plants given fertilizer

Describe the control group = plants NOT given fertilizer

Example #2:

Problem:

Determine the relationship between the amount of salt in the water and the amount of food the fish eat.

Independent Variable = the amount of salt in the water

Dependent Variable = the amount of food the fish eat

Constants = type of fish, temperature of water, type of food, amount of water

(EVERYTHING should be constant except the independent and dependent variables!)

Describe the experimental group = salt added to water

Describe the control group = no salt in the water

Example #3:

Problem:

Determine the relationship between the height of a tree and the amount of sunlight the tree receives each day.

Independent Variable = the amount of sunlight the tree receives each day

Dependent Variable = the height of the tree

Constants = type of tree, amount of water, type of soil, air temperature

(EVERYTHING should be constant except the independent and dependent variables!)

Describe the experimental group = receives sunlight

Describe the control group = no sunlight

Example #4:

Problem:

Determine the relationship between the number of anti-baldness pills a person takes and the number of hairs grown.

Independent Variable = the number of anti-baldness pills a person takes

Dependent Variable = the number of hairs grown

Constants = gender of person, age of person, all start bald, all in good health

(EVERYTHING should be constant except the independent and dependent variables!)

Describe the experimental group = take the anti-baldness pills

Describe the control group = take placebo pills

 

A placebo is a sugar pill or fake pill used in experiments with humans to see if the actual medicine works and to check for side-effects.  The placebo is always given to the control group, since they are the group used for comparison.