Make It
a Fair Test
Akshara has a new “lucky” pen. Last week, with the old pen, Akshara scored 6 out of 10 in a maths test. This week, with the lucky pen, the score was 9 out of 10. Does that prove that the pen is “lucky”?
💡 The Idea
No! Lots of things were different between the two tests, not just the pen. Perhaps this week’s test was easier. Perhaps Akshara studied harder, or slept better, or liked this week’s topic more. When many things change at once, you can’t tell which one made the difference.
A fair test changes only one thing and keeps everything else the same. Then, if the result changes, you know what caused it.
The tools of a fair test
- Compare. Test the “special” thing side by side with an ordinary thing. Scientists call the ordinary one the control.
- Keep everything else the same. The same place, the same time, the same amounts, the same way of measuring.
- Close the gaps. Think about how the result could be cheated, even by accident, and block every way.
- Repeat. One result could be chance. Do it again, and again.
A real miracle would pass a fair test easily. A trick usually can’t, because a fair test takes away the thing the trick depends on.
Change one thing. Keep everything else the same.
Then compare.
🔗 Where You’ve Seen It
- Activity 4: The 1995 Milk Miracle
An ordinary steel tumbler “drank” too. That was the control: if an ordinary object does the same thing, the statue isn’t special. - Activity 8: The “Mid-Brain” Mystery
Cotton pads closed the gaps beside the nose. With the leak blocked, the “power” disappeared. - Activity 3: Ramar’s Herbal Petrol
Hot water and cold water, a secret stirrer and a clear straw: change one thing at a time, and see when the oil appears. - Activity 1: The Balloon That Won’t Burst
A taped side, the dark top and a plain side: three balloons, three results, and only one thing different each time. - Activity 5: The Magic Banana
The villagers brought their own banana. When the healer couldn’t prepare it in advance, there was no magic. - Activity 9: The Idol That Rose From the Earth
Three cups: seeds with water, seeds without water, and water without seeds. Only one thing differs each time, so you can tell exactly what lifted the toy.
🌍 Use It Everywhere
An advertisement says that a soap makes clothes “twice as white”. Twice as white as what? Washed in the same water, for the same time, with the same dirt? Without a fair comparison, the claim tells you nothing.
You use fair tests at school too. To find out whether plants need sunlight, you grow two plants in the same pots, with the same soil and the same water, and put only one of them in the dark.
A shop sells a “power bracelet” and says it makes you run faster. Design a fair test to find out whether it works.
Show the answer
Run the same distance, on the same track, at the same time of day, several times with the bracelet and several times without it, and time each run with a stopwatch. Mix up the order, so that you aren’t always more tired for one of them.
Even better: get an ordinary bracelet that looks exactly the same, and ask a friend to hand you one or the other without telling you which. Why does that help? Find out in Thinking Tool 3!
For parents and teachers
- Vocabulary: this tool introduces the control and the idea of changing one variable at a time, both part of the school science curriculum. A good exercise is to ask children to find the control in each Young Skeptics activity.
- Fair to the claimant: a fair test is fair to both sides. Rationalists such as Abraham Kovoor and B. Premanand insisted on agreeing the test conditions in advance with the person making the claim, so that nobody could later say the test was rigged.
- Going further: discuss why “closing the gaps” matters even with honest people. A test that can be cheated, even accidentally, cannot tell us much.
A Young Skeptics thinking tool, written with the assistance of AI tools, and reviewed by our editors. — CHARVAKA DESK