# 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.

> AQA A-Level Physics (7408) — 3.1.2 Limitations of Physical Measurements · Explain · 5 marks

## Mark scheme (5 marks)

1. Identify 18.3 cm as an anomalous result / outlier
2. The anomalous result should be excluded / not included in the mean
3. Calculate a mean of the remaining results
4. The remaining results are close together / have a small range
5. A small spread / small range indicates high precision

## Key terms

- [precision](https://www.gradenine.co.uk/glossary/precision)

## Related

- [Revision notes for AQA A-Level Physics (7408)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-measures-the-focal-length-daa76f24) · Published by Druglandscape Ltd.