Evaluate validity, anomalies and improvements
Judge repeatability and validity, assess anomalies and propose changes that increase confidence in a specific investigation.
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 does validity ask?
What you will learn
- Distinguish repeatability, accuracy and validity.
- Assess anomalies, range, intervals, controls and measurement quality.
- Propose justified improvements and extensions.
Evaluate this method, not an ideal experiment
Repeatability concerns whether repeated measurements agree. Accuracy concerns how close a measurement is to the true value. Validity asks whether the design tests the intended relationship and supports the conclusion.
Consider whether the independent-variable range and intervals were suitable, repeats sufficient, controls effective and dependent-variable measurement appropriate. Explain how each issue affects confidence.
Improve with a reason
An improvement should address a weakness: increase repeats to estimate variation, calibrate an instrument to reduce bias, use a more precise measure, or standardise a relevant variable.
An anomaly should be checked for a recording or procedural cause. A justified repeated measurement can test it; deleting a value solely because it disagrees with the trend biases the evidence.
Pause and explain
What is the best response to an unexpected single result?
Worked example
A colorimeter is not blanked between samples, and the readings are noisy. Identify two different improvements.
Show the worked solution
- Blank the colorimeter appropriately to improve measurement accuracy.
- Repeat readings to estimate random variation.
- Link each change to the issue it addresses.
Answer Blank the colorimeter with the appropriate reference before readings to reduce systematic measurement bias, and collect repeated readings to assess random variation and calculate a mean.
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 ruler is used to measure a 0.2 mm cell wall. Suggest a more suitable measurement approach and explain why.
Give me a hint
Choose a method with a finer effective scale.
Compare my reasoning
- Use a calibrated microscope scale or a sufficiently resolved image.
- Check the calibration and repeat measurements.
- Explain why the ruler resolution is inadequate.
Use a calibrated eyepiece graticule or a measurable photomicrograph scale; the ruler’s resolution is too coarse for a 0.2 mm feature.
Look for these in your work
- Approach suits scale
- Reason based on resolution
A procedure uses only two concentration values. How does that limit the conclusion and what improvement is suitable?
Give me a hint
Consider whether a trend or shape can be inferred.
Compare my reasoning
- Two values show only a comparison, not the pattern across a range.
- Add several suitable values with useful intervals.
- Repeat each value.
With only two values, the shape of the relationship cannot be assessed well. Use at least five values across a justified range and include repeats.
Look for these in your work
- Limitation stated
- Range and replication improved
A method measures reaction rate but leaves temperature uncontrolled. Explain the validity issue and improvement.
Give me a hint
Link the uncontrolled variable to the measured response.
Compare my reasoning
- Temperature can affect enzyme rate independently.
- Use a controlled water bath and allow samples to equilibrate.
- Keep the same timing and volumes.
Uncontrolled temperature could explain rate differences, so the method may not isolate the tested factor. Use a controlled water bath, equilibrate samples and keep timing and volumes constant.
Look for these in your work
- Confound identified
- Specific control proposed
Common mistakes
- Using “more accurate” without saying how.
- Confusing an extension with an improvement.
- Removing anomalous values without investigating them.
What can you explain now?
What does validity ask?
Compare with the explanation
Whether the design tests the intended relationship
Validity depends on the question, controls and measurements.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
How do repeatability, accuracy and validity describe different qualities of evidence?
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.