A fair test

The idea

In a fair investigation only one thing is changed at a time, while all the other conditions are kept the same.

Three puris, many differences

Riya wants to know if the oil temperature changes how a puri puffs. She makes three puris, but they differ in more than the oil. (Hot oil is dangerous: we only imagine this trial.)

Suppose puri C puffs differently from A and B. Can Riya be sure the oil temperature caused it?

• Yes, C had the hottest oil.
• No, other things were different too.
• I cannot tell.

Make the test fair

Only the oil temperature should differ (it is locked). Tap the boxes to change the other rows until each row is the same for all three puris.

One thing at a time

In a fair investigation, change only one thing at a time. Keep all the other conditions the same.

To test boiling hot, hot and not very hot oil, use dough circles of the same thickness and drop them in the same way.

• Unfair test | Fair test
• What differs | Oil, thickness, flour, size and way of dropping | Only the oil temperature
• If the puris puff differently | Any of the differences could be the reason | The oil temperature must be the reason

Notes

In a fair investigation only one thing is changed at a time, while all the other conditions are kept the same.

Check yourself

Aman wants to test whether the kind of flour changes how a puri puffs. Look at his two puris. What is wrong with his test?

Sana wants to know if the amount of water changes how tall bean plants grow. Which setup is a fair test?

A student changes 3 things at once: thickness, flour and oil temperature. Then the puri puffs better. How many of these 3 things could be the reason?

Answer: 3

All 3 could be the reason, so the test cannot tell us which one mattered. A fair test changes only one thing.

• Nothing is wrong with it.. Look at the thickness row. It is not the same for both puris.
• The oil temperature should differ too.. No. The oil is the same for both, and that is good. Only the flour should differ.
• The thickness differs too, so we cannot tell what caused a difference. — correct. Yes. A thin atta puri and a thick maida puri differ in two things at once.
• Plant 1: lots of water, sunny window. Plant 2: little water, dark cupboard.. Two things differ: the water and the light. We could not tell which one mattered.
• Plant 1: lots of water, big pot, rich soil. Plant 2: little water, small pot.. The pot and the soil differ too, along with the water.
• Plant 1: lots of water. Plant 2: little water. Same seeds, same soil, same pot, same sunlight. — correct. Yes. Only the amount of water is different.