Manipulated variable

In the syllabus and the exam

Identifying the manipulated variable correctly is the starting point for almost every Paper 3 practical and every Paper 2 structured question built around an experiment, because the responding variable, the controlled variables and the hypothesis are all defined in relation to it. Candidates are expected to state the manipulated variable with its unit and, where relevant, the specific values used, such as 'concentration of sucrose solution (0%, 5%, 10%, 15%, 20%)' rather than just 'concentration'. A common mistake is swapping the manipulated variable with the responding variable, for instance naming 'the rate of photosynthesis' as the manipulated variable in an experiment that actually varies light intensity; examiners penalise this because it reverses the cause-and-effect relationship the experiment is designed to test. Another frequent slip is naming a controlled variable, such as temperature, as the manipulated variable when temperature was in fact being held constant across all set-ups. Because the manipulated variable is the one factor the experimenter deliberately changes, it must always appear on the x-axis of any graph plotted from the results, with the responding variable on the y-axis; getting the axes reversed usually traces back to confusion over which variable was actually manipulated in the first place. Because the same short list of manipulated variables recurs across most Form 4 practicals (concentration, temperature, light intensity, pH and time), candidates who memorise the correct unit and typical range for each one tend to answer this part of a structured question fastest and with the fewest careless slips.

Example

In an experiment on the rate of enzyme reaction, the concentration of hydrogen peroxide is the manipulated variable, deliberately set at 1%, 2%, 3% and 4% while every other condition is kept the same.

Related terms

Responding variableControlled variableHypothesis

Source:SRC-DSKP-EN