Controlled (fixed) variable

Malay: Pemboleh ubah dimalarkan (tetap) · Chinese: 固定变量

A controlled variable, also called a fixed variable, is any quantity that is deliberately kept constant throughout an experiment. Keeping these variables steady is what makes the test fair: it ensures the responding variable changes only because of the manipulated variable and not for some other reason. Unlike the manipulated and responding variables, of which there is one each, an experiment usually has several controlled variables, and naming them is a standard part of a scientific-skills answer in the SPM Chemistry papers (4541).

Suppose you are investigating how the concentration of acid affects reaction rate. You change the concentration on purpose (manipulated) and measure the time to react (responding). To be fair, you must keep the temperature, the volume of acid, and the mass and size of the marble the same every time, these are the controlled variables. If, say, the temperature were allowed to drift, you could not tell whether a faster reaction was caused by the concentration or by the warmth.

The confusion to avoid is labelling a controlled variable as the manipulated variable. Remember: you change only one thing (manipulated) and hold constant everything else that could affect the result (controlled). A second slip is forgetting to control a variable that clearly matters, which makes the test unfair and the conclusion unreliable. In an answer it is not enough to write “keep everything else constant”, the examiner wants the actual quantities named, such as temperature, volume of acid and mass of marble.

Controlled variables are the third member of the variable trio and the direct reason a fair test works. Our teachers train you to list every variable that must be fixed, so no easy marks slip away in the exam.