The Scientific Method for Kids: Independent, Dependent and Controlled Variables

The scientific method is less a fixed list of steps and more a way of making sure an answer can be trusted. Its heart is the fair test: change one thing, measure what happens, and keep everything else the same.

The usual steps

• Ask a question you can test.
• Research what is already known.
• Form a hypothesis — a testable prediction, often written "If ..., then ..., because ...".
• Test it with an experiment.
• Record and analyse the data.
• Draw a conclusion and share it — including when the hypothesis was wrong.

Real scientists loop back and forth between these steps, but the order is a useful starting point for students.

The three kinds of variable

• Independent variable — the one thing you change on purpose.
• Dependent variable — what you measure; it depends on the change.
• Controlled variables — everything you keep the same so the test is fair.

A memory trick: the independent variable is the one I change. The dependent variable depends on it.

Example: which paper towel absorbs the most water?

• Question: Which brand of paper towel holds the most water?
• Hypothesis: If we test three brands, then the thickest will hold the most water, because it has more material to soak it up.
• Independent variable: the brand of paper towel.
• Dependent variable: how much water each towel holds (in mL).
• Controlled variables: the size of each towel, the amount of water in the cup, how long the towel stays in the water, and how long it drips before measuring.

Dip each towel for 10 seconds, let it drip for 10 seconds, then squeeze it into a measuring cylinder. Repeat three times for each brand and take the average.

Why repeat trials?

One result can be a fluke — a towel torn at the corner, a drop spilled. Repeating a test and averaging the results makes the conclusion more reliable. If the repeats disagree a lot, that's a sign something isn't being controlled.

Spot the problem

Students learn a lot by finding the flaw in a weak experiment:

• "Lena tested whether music helps plants grow. One plant was by the window with music; the other was in a closet without music." (Two things changed: music and light.)
• "Omar tested which ball bounces highest by dropping each one from whatever height felt right." (The drop height wasn't controlled.)

Observation vs. inference

An observation is what you notice with your senses: "The grass is wet." An inference is an explanation you reason towards: "It probably rained last night." Good scientists keep the two separate in their notes.

In NGSS, planning and carrying out fair investigations is one of the eight science and engineering practices and appears at every grade, from simple tests in Grade 2 to controlled experiments in middle school.