Responding variable

In the syllabus and the exam

The responding variable is what the experimenter actually measures or counts as the outcome of an investigation, and stating it precisely, with a correct unit, is graded separately from stating the manipulated variable in Paper 2 structured questions and Paper 3 practicals. Acceptable answers usually name a measurable quantity, such as 'the time taken for the mixture to turn from blue-black to colourless' or 'the number of bubbles released per minute', rather than a vague outcome like 'the reaction'. A common error is describing the responding variable as a general phenomenon (for example, writing 'photosynthesis' instead of 'the rate of photosynthesis, measured as the volume of oxygen gas collected in a set time') without specifying how it is quantified, which loses marks because the examiner cannot tell how the result would actually be recorded on a table or graph. Candidates sometimes also switch the responding variable with the manipulated variable, especially in experiments where the cause is not obviously physical, such as investigating how temperature affects enzyme activity: here the responding variable is the rate of the enzyme-substrate reaction, not the temperature itself. On a graph, the responding variable is always plotted on the y-axis, against the manipulated variable on the x-axis. Candidates who struggle to identify the responding variable in an unfamiliar context are usually helped by first asking what the experimenter is actually recording in the results table, since that column heading almost always names the responding variable directly. Candidates should also note that a poorly worded responding variable, such as one missing its unit of measurement, is treated the same as an incorrect answer in most SPM mark schemes, even when the correct quantity is implied.

Example

When testing how light intensity affects the rate of photosynthesis, the number of oxygen bubbles produced per minute by Hydrilla is the responding variable, recorded for each light intensity tested.

Related terms

Manipulated variableControlled variableInference

Source:SRC-DSKP-EN