Variables & Controls

Every fair experiment has three kinds of variables. Knowing which is which is the key to designing a test you can actually trust. If you can label the variables clearly, you can tell what caused a result and what was just along for the ride.

The Three Variables

• Independent variable — the ONE thing the scientist changes on purpose. There is only ever one per fair test.
• Dependent variable — what the scientist measures or counts to see the effect of the change.
• Controlled variables — every other condition that is kept exactly the same in every trial.

Why Controls Matter

A control is a baseline you can compare against. Without controls, you cannot tell whether a change happened because of your independent variable or because of something else that drifted by accident. Controlled variables are how scientists rule out other explanations.

• Same materials in every trial (same brand, same size).
• Same starting amount (same volume, same number of seeds).
• Same surroundings (same room temperature, same light).
• Same timing (measure after the same number of minutes).

Worked Example: Bouncing Balls

Question: does the height a ball is dropped from change how high it bounces? You drop the same tennis ball from 50 cm (about 20 in), 100 cm, and 150 cm onto the same hard floor and measure the bounce each time.

• Independent: the drop height (the ONE thing you change).
• Dependent: the bounce height (what you measure).
• Controlled: same ball, same floor, same room, same person dropping, no spin.