Write a reproducible practical method
Sequence a method so another student could vary, measure and control the relevant factors consistently.
Cambridge International Biology 9700, 2028–2030. Paper 5 assesses planning, analysis and evaluation. These original practice tasks are not Cambridge exam questions or mark schemes.
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
What is the purpose of a reproducible method?
What you will learn
- Describe how to vary and measure both variables.
- Include repeats, range, intervals and standardised conditions.
- Write steps in a logical, reproducible order.
Be specific enough to repeat
Describe the apparatus, volumes or quantities where relevant, the independent-variable values, how the dependent variable is measured and when readings are taken. A method should be clear enough for another person to repeat without filling in important gaps.
Use at least five suitable independent-variable values for a quantitative investigation. State replicates at each value and how results will be combined or compared.
Control the comparison
Name each key variable that could affect the response and state how it is standardised. Include a suitable control where it helps separate the effect of the independent variable.
Order steps so preparation, measurement and recording are clear. Do not include actions that do not answer the stated question.
Pause and explain
Which method detail best supports repeatability?
Worked example
Plan an investigation of temperature on catalase rate using a foam-height endpoint.
Show the worked solution
- Use at least five temperatures and repeats at each.
- Standardise catalase and hydrogen peroxide volumes and concentrations.
- Equilibrate, combine, time identically, measure foam height and record.
Answer Use at least five controlled temperatures with repeated trials. Equilibrate equal volumes of catalase and hydrogen peroxide to each temperature, combine them, time the same interval, measure foam height with the same ruler and record all readings. Keep concentrations, volumes and mixing procedure constant.
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.
Improve “add enzyme and see what happens” into a measurable method outline.
Give me a hint
Name quantities, conditions, measurement and repeats.
Compare my reasoning
- Specify enzyme and substrate volumes and concentrations.
- Use a defined range of one varied condition.
- Measure a stated response at fixed intervals and repeat trials.
Specify equal enzyme and substrate volumes, vary one stated factor across at least five values, measure a defined response such as time to a colour endpoint, repeat each condition and record results consistently.
Look for these in your work
- Variation specified
- Response operationally defined
- Repeats and controls addressed
An experiment varies temperature but measures each sample after a different time. Why is that a problem and how should it be fixed?
Give me a hint
Time can affect the dependent variable independently.
Compare my reasoning
- Identify time as an uncontrolled variable.
- Use the same reaction interval or a consistent rate calculation for every temperature.
Different reaction times confound temperature with duration. Use the same measurement interval for each sample or calculate rate using a consistent procedure.
Look for these in your work
- Confounding identified
- Specific control proposed
A new context uses a digital colorimeter instead of visual colour matching. What should the method specify?
Give me a hint
Make the instrument reading repeatable.
Compare my reasoning
- Select a consistent wavelength or instrument setting if the instructions require it.
- Use the same blank, cuvette orientation and timing.
- Record absorbance with instrument precision.
Specify the consistent instrument setting, blanking procedure, cuvette handling and timing, then record absorbance at a precision supported by the display.
Look for these in your work
- Instrument method described
- Consistency and precision addressed
Common mistakes
- Writing “measure the rate” without describing what is measured and over what time.
- Using different reagent volumes across conditions.
- Failing to say how a variable is kept constant.
What can you explain now?
What is the purpose of a reproducible method?
Compare with the explanation
Another person can follow it and collect comparable measurements
Specific steps and conditions support repeatability.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
What details make a method reproducible, and why is one changed factor easier to interpret?
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.