A control experiment is the standard for the experiment. This means that the results of the project experiment are compared to the results of the control.
Since changes in the project experiment’s independent variable causes changes in the experiment’s dependent variable, then the independent variable is kept constant (unchanging) in the control experiment. Removing the independent variable is not possible in some experiments. For example, if the position of the Sun is the independent variable, one cannot remove the Sun. Thus, there are two types of control experiments–one without the independent variable and the other with the independent variable. Following are examples of both types of control experiments.
Example 1: Control Experiment without Independent Variable
Experiment Purpose: To determine how the amount of light affects the growth of plants.
independent variable: amount of light
dependent variable: growth of plant
The control experiment would be place a test plant in a dark place. Thus, the independent variable, which is the amount of light will be zero.
Example 2: Control Experiment with Independent Variable
If the independent variable cannot be removed, then you must randomly choose part of the project experiment to be the control experiment. Following are two examples of this type of control.
Experiment Purpose: To determine how the position of the Sun affects the direction of a shadow. FYI: Since during the day the Sun rises in the east and sets in the west, time can be used as the Sun’s position.
independent variable: amount of time
dependent variable: direction of shadows
Since the sun cannot be removed from the experiment, then you randomly choose a time (sun position) for your control experiment. It could be your starting or ending observation time. It could be noon, when the Sun is at its highest position in the sky. The later would be my choice because I have learned that the Sun is lowest in the morning, highest at noon, and then moves down toward the west in the afternoon. You would be comparing shadow directions in the morning and shadow directions in the afternoon to the control which is the results at noon. FYI:: Don’t forget to consider daylight saving time because you want noon to be sun time. During daylight saving time, noon is at 11am.
Experiment Purpose: To determine if the shape of metal affects its magnetic strength.
independent variable: the shape of the metal
dependent variable: the strength of the metal’s magnetism
This is another example of the control being randomly picked.It could be any one of the shapes of metal used in the experiment. But the best choice would either be the smallest, medium, or largest shape.
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