Identifying Variables

The three types of variable

• Manipulated variable, the factor that is deliberately changed by the experimenter across different trials or setups.
• Responding variable, the factor that is measured or observed as the outcome, which may change because of the manipulated variable.
• Controlled variables, all other factors that could affect the result but are kept the same throughout the experiment, so that any change in the responding variable can be attributed only to the manipulated variable.

How to identify each variable in a question

• Find what the experimenter sets to different values on purpose, such as different temperatures, concentrations, or light intensities, this is the manipulated variable.
• Find what is recorded or measured as the result, such as the rate of a reaction, the height of a plant, or the number of bubbles produced, this is the responding variable.
• List the factors that must be kept the same across every setup being compared, such as the volume of solution, the species of organism, or the time allowed, these are the controlled variables.

Worked example

A student investigates the effect of temperature on the rate of photosynthesis in Hydrilla, measuring the number of oxygen bubbles produced per minute at 20°C, 30°C, and 40°C, while keeping the light intensity and the concentration of sodium hydrogen carbonate solution constant.

Manipulated variable: temperature.

Responding variable: rate of photosynthesis, measured as the number of oxygen bubbles produced per minute.

Controlled variables: light intensity and concentration of sodium hydrogen carbonate solution.

Comparing the three variables

Identify the manipulated variable first, then the responding variable, then list the controlled variables.

Another worked example

A student investigates the effect of pH on the activity of the enzyme amylase, timing how long it takes for starch to be fully digested at pH 4, pH 7 and pH 9, while keeping the temperature and the concentration of amylase the same.

Manipulated variable: the pH of the mixture.

Responding variable: the time taken for the starch to be fully digested.

Controlled variables: the temperature and the concentration of amylase.

How it is examined

Identifying variables is tested directly in Paper 3 and in the data-response questions of Paper 2. A question describes an investigation and asks you to name the manipulated variable, the responding variable, and the controlled variables, sometimes with their units.

Marks are awarded for naming the exact measured quantity rather than a vague factor, and for listing enough controlled variables to keep the test fair.

Paper 3-style questions

Question 1 (identify variables). A student investigates how light intensity affects the rate of photosynthesis in pondweed by counting bubbles at three distances from a lamp. State the manipulated, responding and one controlled variable.

Model answer. Manipulated variable: the light intensity (distance from the lamp). Responding variable: the number of gas bubbles per minute.

Controlled variable: the temperature of the water (also the type and length of pondweed).

Question 2 (operational definition). In the same experiment, how is the rate of photosynthesis measured operationally? Model answer. The rate of photosynthesis is measured as the number of gas bubbles released per minute from the cut stem of the pondweed.

Question 3 (controlled variables). Give two controlled variables and explain why each must be kept constant. Model answer. The temperature must be kept constant because it also affects the rate of photosynthesis; the concentration of sodium hydrogen carbonate must be kept constant because carbon dioxide is a raw material.

Keeping them constant ensures that any change in bubble rate is due only to light intensity.

Question 4 (fair test). Explain why only one variable should be manipulated at a time. Model answer. Only the manipulated variable should change, so that any change in the responding variable can be linked to it alone; if two factors changed together, you could not tell which one caused the result, and the test would not be fair.

Common mistakes

Source:SRC-DSKP-EN

Frequently asked questions