Plan a practical investigation
Turn an unfamiliar biology investigation into a fair, measurable plan with a useful range of values.
Cambridge International Biology 9700, 2028–2030. Paper 3 assesses practical skills in a supervised laboratory; these lessons support preparation but do not replace safe, supervised laboratory experience.
Before you begin
- Use biological vocabulary and basic calculations from the AS Biology course. Follow your school laboratory rules and teacher instructions for all practical work.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which variable is deliberately changed by the investigator?
What you will learn
- Identify independent and dependent variables.
- Choose at least five suitable values and intervals for a quantitative investigation.
- Select controls, repeats and standardised variables.
Make the question testable
The independent variable is deliberately changed; the dependent variable is measured. State the biological relationship in a way that links them. Choose a range wide enough to reveal a pattern and at least five distinct values for a quantitative investigation.
Keep key background conditions constant so the independent variable is the main planned difference. Choose a measurement that answers the question, and include repeats so results can be compared rather than relying on one reading.
Controls and standardisation
A control helps identify whether the planned treatment caused the measured change. Standardise other variables that could meaningfully affect the dependent variable, and explain how each will be kept constant.
An appropriate plan says how the independent variable is changed, how the dependent variable is measured, how many repeats are collected, and which variables are controlled.
Pause and explain
What minimum number of different independent-variable values should a Paper 3 investigation normally plan?
Worked example
A student tests how pH affects amylase activity. Name the variables and outline a suitable range.
Show the worked solution
- Change pH and measure reaction time or rate with a stated endpoint.
- Use at least five pH values across a biologically useful range.
- Keep temperature, enzyme concentration, substrate concentration and total volumes constant; repeat each pH.
Answer Independent variable: pH. Dependent variable: amylase activity measured by time to a defined endpoint or a calculated rate. Use at least five pH values, repeats and standardised volumes, concentrations and temperature.
From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A student investigates how light intensity affects photosynthesis rate. Identify the variables and two conditions to standardise.
Give me a hint
Separate the deliberately changed factor from the measured response.
Compare my reasoning
- Change light intensity using a measured distance or intensity setting.
- Measure oxygen production using one defined method and time interval.
- Keep plant species, temperature, carbon dioxide availability and measurement time consistent.
Independent variable: light intensity. Dependent variable: photosynthesis rate estimated from oxygen produced per unit time. Standardise temperature and carbon dioxide availability, among other relevant conditions.
Look for these in your work
- Both variables identified
- Measurement is defined
- Two relevant controls explained
Design five concentrations between 0 and 1.0% for a concentration investigation, including both endpoints.
Give me a hint
Use equal intervals and include five distinct values.
Compare my reasoning
- Divide the total range into four equal intervals.
- Use 0.0%, 0.25%, 0.50%, 0.75% and 1.00%.
A suitable five-value series is 0.00%, 0.25%, 0.50%, 0.75% and 1.00%.
Look for these in your work
- At least five values
- Range and intervals are clear
A transfer question changes the specimen and asks whether the same treatment effect holds. What should be retained and what should be reconsidered?
Give me a hint
Keep the logic of the design, but revisit biological controls.
Compare my reasoning
- Retain a measurable response, repeats and a suitable range.
- Reconsider conditions that depend on the new specimen, such as size or surface area.
- Explain how the comparison would remain fair.
Retain the clear variables, range, measurement and repeat structure, but adapt and justify specimen-specific controls so the new comparison remains fair.
Look for these in your work
- Transfer of method explained
- New context-specific control identified
Common mistakes
- Using only two independent-variable values when a pattern across a range is needed.
- Saying “keep conditions the same” without naming how.
- Treating repeats as a substitute for controlling variables.
What can you explain now?
Which variable is deliberately changed by the investigator?
Compare with the explanation
The independent variable
It is the factor deliberately varied.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
How do the independent and dependent variables differ, and why should a practical use a range and repeats?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.