Biology editorial review is supported by the
named review panel.
Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
What this skill means
The independent variable is deliberately changed, the dependent variable is measured, and controlled variables are kept constant. A control experiment is a separate comparison that removes the factor being tested.
Variable questions appear throughout Papers 5 and 6. They are easy marks only when the answer names a measurable quantity rather than using vague words such as “amount” or “keep everything the same”.
Independent, dependent and controlled variables
The independent variable is the factor deliberately changed, such as temperature in an enzyme investigation. The dependent variable is the outcome measured, such as time for starch to disappear. Controlled variables are kept constant so any change in the result can be attributed to the independent variable.
Name the quantity and, where useful, how it is measured. “Amount of enzyme” is weaker than “volume and concentration of amylase solution”.
A control experiment is not a controlled variable
A control experiment is a separate comparison set-up in which the factor being investigated is absent or inactive. For example, a boiled-enzyme tube shows whether an observed change requires an active enzyme. Controlled variables, by contrast, are conditions kept the same across all test set-ups.
How to write a fair-test answer
Use the pattern: “Keep [named quantity] the same by [method].” Examples include using the same volume measured with a pipette, keeping pH constant with a buffer, or using cylinders cut to the same length and diameter.
Do not list the independent variable among the controls. Do not say “repeat to make it fair”; repeats improve reliability, while controlled variables make the comparison fair.
Use this before submitting your answer
A complete answer is specific enough for another person to repeat or assess.
✓State exactly what is changed.
✓State exactly what is measured and the unit.
✓Name at least two relevant controlled variables.
✓Explain how each controlled variable is kept constant.
✓Use precise quantities such as volume, mass, concentration, length and time.
Try the skill in a question (4 marks)
A student investigates the effect of temperature on amylase. State the independent variable, dependent variable and two controlled variables.
Show the answer and marking guidance
Independent variable: temperature of the reaction mixture.
Dependent variable: time taken for starch to disappear or rate calculated as 1 ÷ time.
Controlled variable: pH, kept constant using the same buffer.
Controlled variable: volume or concentration of amylase or starch, measured identically each time.
Original practice content written for this site. It is not an official Cambridge question.