1.5 A Fair Test: Change Only One Thing at a Time

Suppose you want to know whether hotter oil makes a puri puff faster. You fry one puri in very hot oil and another in cooler oil — but you also make the first one thinner and drop it at a different angle. The hot one puffs better. Why? Was it the heat? The thinness? The angle? You cannot tell. By changing three things at once, you ruined your own experiment.

The Golden Rule of a Fair Test Change one variable at a time and keep all the others the same. Only then can you be sure that any difference you see was caused by the one thing you changed. A test that follows this rule is called a fair test (or a controlled experiment).

Worked Example 1

Designing a fair test for: “Does hotter oil make a puri puff faster?”

• Change (independent variable): the oil temperature — try three levels: not very hot, hot, boiling hot.
• Measure (dependent variable): the time in seconds for the puri to puff.
• Keep the same (controlled variables): same flour, same dough thickness, same diameter, dropped the same way, same pan.
• Method: roll three identical circles, heat the oil to the first temperature, slide one in, time the puffing; repeat for the other two temperatures.

Because only the temperature changed, any difference in puffing time can fairly be blamed on the temperature — exactly what we wanted to find out.

A Common Trap “I changed the oil temperature and used a thinner puri, and it puffed faster — so hotter oil works!” This is wrong reasoning. Two things changed, so the experiment cannot say which one mattered. When more than one variable changes at once, the test is unfair and its result proves nothing.

Worked Example 2

Spot the unfair test. Reena wants to find out if plants grow taller with more sunlight. She puts Plant A on a sunny windowsill and waters it daily, and keeps Plant B in a dark cupboard but forgets to water it. After a week, Plant A is taller. What is wrong?

Answer: Two variables changed at once — sunlight and water. Plant A had both more light and more water, so we cannot say whether it grew taller because of the light, the water, or both. A fair test would give both plants the same water and change only the sunlight.