Variables: How do we test variables in a controlled experiment?

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Variables are things that vary/change what happens in an experiment.

Scientist conduct experiments to find what causes or effects something. To find out the cause or effect, scientists conduct experiments, testing one thing at a time. A variable is anything that can change (vary) the outcome of an experiment. Scientists test only one variable at a time, and control for (keep the same) the other variables. The independent variable is what I change in the experiment. The dependent variable depends on the independent variable, and is what I observe at the afterwards. An experiment that changes only one variable at a time is called a controlled experiment. Experiments also have controlled variables. Controlled variables are what we keep constant (the same), and we observe them carefully too.

Example of variables in from daily life:

Independent variables that affect student test scores:


+hours of studying

+amount of sleep

+paying attention in class


Independent variable: The number of hours students studied for a test

Dependent variable: Student performance on the test

Variables to control: Studying from the right book (for example, don't study science from the social studies book), attending class, amount of sleep students get, etc.

Independent variables that we tested one at a time included:

1. mass (the weight of pennies)

2. release position (the angle from which we dropped the pendulum)

3. length of string

The dependent variable was the number of cycles (swings).

The length of string was the only independent variable that had an effect on the dependent variable. The greater the length of the string, the fewer was the number of cycles. This represents an inverse (opposite) relationship.

We graphed results, with the independent variable (x) of length on the horizontal line, and the dependent variable (y), the number of cycles, on the vertical line. The graph showed the relationship in a visual way.

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We then tested one variable at a time with an investigation on variables that affect the buoyancy (the ability to float) of a boat.

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Independent variables that can affect buoyancy include:

1. Roughness of the water

2. Distribution of the penny passengers

3. Placement of the passengers

4. Capacity (size) of boat

The dependent variable was the number of passengers (pennies) a boat could hold and stay afloat.

We controlled for all variables except for the capacity. To control for the other variables, we made sure to avoid shaking the water, to distribute passengers evenly (not only on one side of the boat), and to place passengers gently.

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We then graphed the results, and drew a line through the average of the points. We found that the greater the capacity of a boat (independent variable x), the greater the number of passengers (dependent variable y) it could hold. From the graph, you can predict the number of passengers a boat can hold by extrapolating (extending the line based on the slope) from the graph.


1. What is a controlled experiment?

2. What was the independent variable we tested and graphed in the pendulum experiment, and how did it affect the dependent variable?

3. What was the independent variable we tested and graphed in the boat experiment, and how did it affect the dependent variable?

4. In the boat experiment, how did we control for the other variables, in order to test one variable at time? Give examples.

5. Why is it important to only test one variable at a time?

6. Based on the results of the boat experiment, what conclusion can you draw regarding the relationship between boat capacity and number of passengers it can hold without sinking?


1. Video: Data Collection and Variables

2. Science, Grade 5 (Spectrum Science)

3. National Geographic for Kids

4. Website on variables and sample science projects

5. Science News for Kids

6. Variables Unit Summary

7. How to Build A Catapult