What is a fair test?
Imagine two friends racing toy cars. One car starts at the top of a steep ramp. The other starts halfway down a gentle ramp. The first car wins, but was it really the faster car? We can’t tell, because too many things were different.
A fair test changes only one thing and keeps everything else the same. That way, if something different happens, you know what caused it.
Variables
A variable is anything in an experiment that can change. In a fair test, there are three kinds of variables:
| Kind of variable | What it means | Plant example |
|---|---|---|
| The one you change | The thing you choose to make different | Amount of sunlight |
| The one you measure | What you watch to see what happens | How tall the plant grows |
| The ones you keep the same | Everything else | Type of seed, soil, pot size, water |
Scientists also call these the independent variable, the dependent variable and the controlled variables. You’ll use those names in later grades.
Planning step by step
Here is how to plan a fair test.
- Ask a question. Does the height of a ramp change how far a car rolls?
- Choose what to change. The height of the ramp.
- Choose what to measure. How far the car rolls after the ramp, in centimetres.
- List what to keep the same. The same car, the same floor, the same starting spot, no pushing.
- Make a prediction. I think a higher ramp will make the car roll farther.
- Plan your table. Make columns for ramp height and distance.
Doing several trials
Something odd can happen in any single try. The car might hit a crumb or wobble off course. That’s why scientists do more than one trial. If you test each ramp height three times, you can find the average and trust your result more.
| Ramp height | Trial 1 | Trial 2 | Trial 3 | Average |
|---|---|---|---|---|
| 10 cm | 52 cm | 48 cm | 50 cm | 50 cm |
| 20 cm | 95 cm | 101 cm | 98 cm | 98 cm |
| 30 cm | 140 cm | 146 cm | 143 cm | 143 cm |
The results show a clear pattern: higher ramps made the car roll farther.
Spotting an unfair test
Read this plan: "I’ll water one plant with tap water in a sunny window and another with juice in a dark closet." What’s wrong? Two things are changing, the liquid and the light. If one plant dies, we won’t know why. To fix it, put both plants in the same place and change only the liquid.
In the real world
Companies use fair tests all the time. Paper towel makers test different towels with the same amount of water to see which soaks up the most. Shoe makers test soles on the same surface to see which grips best. Farmers test new seeds next to old seeds in the same field.
Common mistakes
- Changing two things at once.
- Doing only one trial.
- Measuring in different ways, such as using a ruler one time and guessing the next.
Try it yourself
Plan a fair test to answer: "Which paper towel brand soaks up the most water?"
- What will you change?
- What will you measure?
- What will you keep the same? List at least four things.
- How many trials will you do?
Quick recap
- A fair test changes only one thing at a time.
- A variable is anything that can change.
- You change one variable, measure one, and keep the rest the same.
- Several trials make your results more trustworthy.
- If two things change at once, you can’t tell what caused the result.