Controlled variable

In the syllabus and the exam

Controlled variables are every factor other than the manipulated variable that could affect the responding variable, and they must be held constant so that any change observed can be attributed to the manipulated variable alone; this is what makes an experiment a fair test. In Paper 2 and Paper 3, candidates are usually asked to list at least two or three controlled variables relevant to the specific experiment, along with how each one is kept constant, for example 'the same volume, 10 cm3, of hydrogen peroxide solution is used in each test tube' rather than simply writing 'volume'. Marks are lost when a candidate lists a variable without stating how it is controlled, or when the variable named is actually the manipulated or the responding variable in disguise. A frequent error in enzyme and photosynthesis experiments is forgetting temperature as a controlled variable, since it is easy to focus only on the variable being changed and overlook the fact that an uncontrolled temperature could itself alter the reaction rate and confound the result. Controlled variables are conceptually different from a control set-up (a set-up with no manipulated variable applied at all, used for comparison), and the two terms should not be used interchangeably in an answer. Examiners increasingly reward candidates who explain the consequence of failing to control a variable, not just list it, so a stronger answer adds a short reason such as 'if temperature is not kept constant, any change in reaction rate could be caused by temperature rather than by the manipulated variable'.

Example

In an experiment comparing germination rates at different temperatures, the volume of water given to each seed, the type of seed used, and the light conditions are all controlled variables kept the same across every set-up.

Related terms

Manipulated variableResponding variableHypothesis

Source:SRC-DSKP-EN