A student measures the focal length of a converging lens by finding the distance at which it focuses light from a distant window onto a screen. She repeats the measurement six times. Her results are: 12.4 cm, 12.6 cm, 12.5 cm, 18.3 cm, 12.4 cm, 12.5 cm. Explain how the student should process these results to obtain a reliable value for the focal length, and explain what the spread of the remaining results tells you about the precision of her measurements.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
1. Identify 18.3 cm as an anomalous result.
2. Exclude the anomalous result from further calculation.
3. Calculate the mean of the remaining five results: (12.4+12.6+12.5+12.4+12.5)/5 = 12.48 cm.
4. Note that the remaining results are close together, giving a small range of 0.2 cm.
5. A small spread indicates that the measurements are highly precise.
2. Exclude the anomalous result from further calculation.
3. Calculate the mean of the remaining five results: (12.4+12.6+12.5+12.4+12.5)/5 = 12.48 cm.
4. Note that the remaining results are close together, giving a small range of 0.2 cm.
5. A small spread indicates that the measurements are highly precise.
Examiner tips
- Use the word ‘anomalous’ or ‘outlier’ to show you recognise the 18.3 cm value. Show the calculation of the mean and state the unit. Mention the small range or spread explicitly. Link the small spread to high precision.
- Ensure you exclude the outlier before computing the mean – including it would give a wrong value.
Common mistakes
- Including the 18.3 cm value in the mean. Failing to state the unit (cm) for the mean. Confusing precision with accuracy – not recognising that a small spread indicates precision.
Mark scheme (5 marks)
- Identify 18.3 cm as an anomalous result / outlier
- The anomalous result should be excluded / not included in the mean
- Calculate a mean of the remaining results
- The remaining results are close together / have a small range
- A small spread / small range indicates high precision
Key terms in this question
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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