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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
What this skill means
A strong investigation plan states a range of independent-variable values, explains exactly how the dependent variable is measured, controls key conditions, repeats each value, calculates a mean and says how results will answer the question.
Planning questions feel open-ended, but the mark scheme usually rewards the same ingredients. A numbered method makes each marking point visible and prevents a long paragraph from hiding missing details.
Start with a testable range
Name the independent variable and give a sensible range with at least five values where appropriate. “Use different temperatures” is vague; “test 20, 30, 40, 50 and 60 °C” is testable.
Choose intervals wide enough to reveal a pattern but safe and realistic for the apparatus.
Measure a defined outcome
State the dependent variable, equipment and endpoint. For example: measure the time until iodine no longer turns blue-black, record mass using a balance to 0.01 g, or count oxygen bubbles for one minute.
If the measurement is subjective, explain how it will be standardised or replaced with a more objective method.
Control, repeat and process
Name specific controlled variables and how they are held constant. Repeat every independent-variable value at least three times where practical, identify anomalous results and calculate a mean. State the graph to plot or comparison to make.
Add relevant safety, not generic safety
Give a precaution connected to the actual hazard: use a water bath because ethanol is flammable, wear eye protection when heating Benedict’s solution, or handle glassware carefully. “Be careful” does not earn a mark.
Use this before submitting your answer
A complete answer is specific enough for another person to repeat or assess.
✓Give at least five sensible independent-variable values where suitable.
✓Name equipment and a measurable endpoint.
✓Control at least two relevant variables.
✓Repeat each condition and calculate a mean.
✓State how results will be displayed or compared.
✓Add a relevant safety precaution or control set-up where appropriate.
Try the skill in a question (6 marks)
Plan an investigation to determine how light intensity affects the rate of photosynthesis in pondweed.
Show the answer and marking guidance
Place the lamp at several measured distances or measure light intensity directly.
Measure oxygen volume in a set time using a gas syringe, or count bubbles for a fixed time.
Keep pondweed length or species, carbon dioxide concentration, temperature and time constant.
Allow the apparatus to equilibrate at each light level.
Repeat each level and calculate a mean rate.
Plot mean photosynthesis rate against light intensity and use a heat shield or water bath to control temperature.
Original practice content written for this site. It is not an official Cambridge question.