Fair test

Malay: Ujian adil · Chinese: 公平检验

A fair test is an experiment in which only the manipulated variable is changed while every other variable that could affect the result is kept constant. Because just one thing is changed at a time, any change in the responding variable can be blamed on the manipulated variable alone, and the conclusion about cause and effect is valid. Designing a fair test is one of the most common scientific-skills tasks in the SPM Chemistry papers (4541), so you must be able to say exactly which variable you change and which you hold constant.

For example, to test fairly whether temperature affects how fast magnesium reacts with acid, you change only the temperature. You keep the concentration and volume of the acid, and the length and mass of the magnesium ribbon, the same each time. If two things were changed together, say, a hotter acid that was also more concentrated, the test would not be fair, because you could not tell which change caused the faster reaction.

The confusion to avoid is thinking any experiment is automatically a fair test. It is fair only if the controlled variables really are kept constant; forgetting even one of them makes the result unreliable. Students also mix up a fair test with simply repeating readings, repeating improves reliability, but fairness is about changing one variable at a time. A useful way to check your own plan is to list, in one column, the single variable you change and, in another, every variable you keep the same; if the second column is empty, your test is almost certainly not fair.

Within the scientific method, the idea of a fair test ties the three kinds of variable together and justifies your conclusion. Our teachers give you a simple checklist so you can prove any experiment you describe is fair.