Now it's time to think about how best to set up your science fair experiment to give you accurate and trustworthy results. You should keep in mind both the Scientific Method and the question you are trying to answer.
An experiment is made up of two nearly identical parts--let's say, two trays of tomato plants. The tiny differences that will test your hypothesis are called "variables." Let's look at the three kinds of variables.
The Independent Variable: this is a variable that you intentionally change. In the case of our tomato plant experiment, it would be the brightness of the light in Tray #1.
The Dependent Variable: this is the difference between the two parts of the experiment that happens when the independent variable is changed. In this case, it would be the size of the tomatoes in Tray #1. So you turn the up the lights in Tray #1 (the independent variable) and the tomatoes get bigger (dependent variable).
The Controlled Variables: these are the variables that are kept the same in Tray #2. In other words, when you turn the lights up in Tray #1, the lights in Tray #2 stay the same. So when the tomatoes in Tray #1 get bigger, you can say that the difference in size between the tomatoes in Tray #1 and Tray #2 is the result of the independent variable, or the light that you made brighter.
Tray #2 is called a "control group." This is an identical setup to Tray #1, but it is kept in its natural, unchanged state. Tray #1 is called the "experimental group." So when you change something in the experimental group (like the brightness of the light), the control group stays the same so that you can measure changes in you dependent variable (the size of the tomatoes).
