PSLE Science: 3 Types of Experiment Variables
Part 3 of 3 of the PSLE Science OEQ Guide: build a full experiment answer, using everything from Parts 1 and 2.
Picture this. You're baking cookies for the school funfair. The fifth batch comes out way better than the four before it.
But between tries you changed a few things. A different brand of butter. Less flour. An extra egg.
So which one did it?
You can't tell. And that's more of a problem than it sounds. You've got one butter cookie recipe that works, and no idea why it works. Next month you want to make chocolate chip ones. Where do you start? Back to guessing.
But if you'd changed just one thing each time, you'd know what every ingredient actually does. Then you're not guessing any more. You're building.
How this applies in a science question
A student wants to find out whether a ball bounces higher when it's dropped from higher up.
She drops the same ball from three heights — 50 cm, 100 cm, 150 cm. Same ball every time. Same floor. She lets it go rather than throwing it. Then she measures how high it bounces back.
That's the thing you should have done with the cookies. One difference between attempts. Everything else identical.
What the three things are called
The one thing she set differently was the height she dropped from. That's the changed variable. There's only ever one.
The thing she measured was how high the ball bounced. That's the measured variable. She didn't decide it — that's what she was there to find out.
Everything she kept the same — the ball, the floor, letting go instead of throwing — those are the controlled variables. Quickest way to spot them: they're the things she could have changed, and didn't.
| Variable | What it means | In the bouncing ball experiment |
|---|---|---|
| Changed | The one thing set differently. Only ever one. | The height it was dropped from |
| Measured | What you're trying to find out. Only ever one. | How high it bounced |
| Controlled | Everything held the same. Usually several. | Same ball, same floor, dropped not thrown |
The aim uses two of them. The fairness question is about the ones you kept the same. It's worth the first thirty seconds identifying these carefully.
What a strong answer does
- Names the whole variable, not half of it. "Height" isn't a variable on its own — in the bouncing ball experiment there are two heights, and they're the changed and the measured one. "The height it was dropped from" says which you mean. Write the whole phrase.
- Doesn't mix up changed and measured. One question sorts them. Did the person set this, or find it out?
- Names the thing, not the method. "She dropped the ball from three different heights" says what was done. The answer wanted is the thing itself: the height it was dropped from.
- Says why the controlled variables were controlled. In your kitchen you never have to explain why you kept the flour the same — you already know. On paper you do. "She used the same floor" earns nothing on its own. Add why, and it does.
For more tips on improving your answers—from avoiding common mistakes to using precise science language—explore the complete PSLE Science OEQ Guide.