What Is a Fair Test? Understanding Variables

Asking a good question

Imagine you want to find out: Does a plant grow taller with more water? To answer it properly, you cannot just water one plant a lot and hope. You need to run a fair test. A fair test is the careful way scientists make sure their answer is true and not just luck. The secret is all about something called variables.

What is a variable?

A variable is anything in your experiment that could change or be changed. In a plant experiment, the variables include the amount of water, the size of the pot, the type of soil, how much sunlight there is, and how tall the plant grows.

Scientists sort these variables into three jobs. Getting these jobs right is what turns a messy experiment into a proper, fair test.

The three kinds of variable

1. The thing you change — the independent variable. This is the one thing you decide to change on purpose. In our experiment, it is the amount of water. You might give one plant a little, one plant some, and one plant lots.

2. The thing you measure — the dependent variable. This is what you measure to get your result. Here, it is how tall each plant grows. We call it "dependent" because the result depends on what you changed.

3. The things you keep the same — the control variables. These are all the other variables you must keep exactly the same for every plant: the same pot size, the same soil, the same spot by the window, the same type of seed, and the same start day. Keeping them the same is what makes the test fair.

The golden rule: change only ONE thing

Here is the heart of a fair test: change only one thing at a time.

Why does this matter so much? Suppose you gave one plant more water and a sunnier windowsill, and it grew taller. Which one helped — the water or the sunlight? You would have no idea, because you changed two things. By changing only the water and keeping everything else the same, you know for certain that water was the cause.

Variable	Job in our plant test
Amount of water	Change this (independent)
Height of plant	Measure this (dependent)
Pot, soil, light, seed, day	Keep the same (control)

Try this: a fair plant test

With a grown-up's help, you can run your own fair test.

1. Get three identical pots, the same soil, and three of the same kind of bean seed.
2. Plant one seed in each pot at the same depth.
3. Put all three on the same windowsill.
4. Change one thing: give pot A 1 spoon of water a day, pot B 3 spoons, and pot C 6 spoons.
5. Measure: every few days, use a ruler to record how tall each plant is.
6. Keep everything else the same.

After two weeks, look at your numbers. Which amount of water grew the tallest plant? Because you ran a fair test, you can trust your answer. You can read more about how seeds become plants in The Life Cycle of Plants.

Try this instead: paper aeroplanes

No seeds? Test paper aeroplanes! Change one thing — for example, the size of the wings — and keep everything else the same: the same paper, the same throw, the same launch spot. Measure how far each plane flies. Throw each plane three times and find the average distance. Repeating like this evens out lucky or unlucky throws.

Why fair tests matter

Fair tests are how scientists learn the truth about everything, from which medicine helps people to which crops grow best. Without fair tests, we would just be guessing. Now you know the trick: change one thing, measure one thing, keep the rest the same — and you will get an answer you can trust. This careful thinking is part of the bigger plan scientists follow, which you can explore in How to Be a Scientist: Observe, Predict, Test.