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A variable is any factor, trait, or condition that exists in differing amounts or types. An experiment usually has three kinds of variables: independent, dependent, and controlled. In a good experiment the scientist must be able to measure the values for each variable.

Independent variable: the condition you change during the experiment. In most good science fair projects only one independent variable is changed at a time.

Dependent variable(s): the variable(s) which you observe during the experiment.

Controlled variable(s): the variable(s) which you try to keep the same during the experiment.

Question Independent Variable Dependent Variable(s) Controlled Variable(s)
Does heating a cup of water allow it to dissolve more sugar? Temperature of the water measured in degrees Centigrade Amount of sugar that dissolves completely measured in grams
  • Stirring
  • Type of sugar
  • Amount of water
"More stirring or more water might also increase the amount of sugar that dissolves and different sugars might dissolve differently, so to ensure a fair test I want to keep these variables the same for each cup of water."

For more information and examples of how to identify variables in a science project check out our variables reference page.

1. What is the question you are trying to answer in your science project?
2. What is the independent variable in your science project?
3. Will the independent variable be measured?
If you answered “yes”, describe the units of measurement (grams, degrees Celsius, milliliters etc.).
4. List all of the dependent variables in your project and how you will measure them.
5. List all of the controlled variables in your project.

To self-check whether or not you have done a good job identifying and thinking through your project’s variables, think about the following questions and answer “yes” or “no” honestly.

Is the independent variable measurable?
Can I change the independent variable during the experiment?
Have I identified all relevant dependent variables, and are they all caused by and dependent on the independent variable?
Are all dependent variable(s) measurable?
Have I identified all relevant controlled variables?
Is it possible for me to hold all controlled variables at a steady value during the experiment?

If you answered “no” to any of the self-check questions then your project may not be a good one yet. Go back and:

  1. Figure out how to improve your experimental design to the point that you can honestly answer “yes” to all of the questions.
  2. If you can’t do number 1, you may need to ask your teacher or another adult mentor for help. On rare occasion some of these questions may have a “no” answer even for a very good project. If you think this is the case, be prepared to explain your thinking to your teacher/adult mentor. Otherwise, you may need to consider choosing a different science question for your project.
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Free science fair projects.