# A student measures the diameter of a wire five times using a micrometer. The results are: 0.52 mm, 0.51 mm, 0.52 mm, 0.53 mm and 0.52 mm. Explain how the student can use these repeat measurements to obtain a more reliable value for the diameter and describe ONE source of systematic error that could affect all five readings.

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

## Mark scheme (5 marks)

1. Calculate the mean (average) of the five repeat measurements
2. The mean reduces the effect of random error (accept: gives a more reliable/accurate result because random errors can be above or below the true value)
3. Identify the range of the results (0.51–0.53 mm) or state that the spread of readings indicates the size of random error / uncertainty
4. A systematic error affects all readings in the same direction / by the same amount (e.g. the micrometer is not zeroed correctly / zero error)
5. Taking more repeats / increasing the number of measurements does NOT remove a systematic error — the mean will still be wrong (i.e. systematic error cannot be identified or removed by averaging alone)

## Key terms

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

## 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)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-measures-the-diameter-of-2aa1754b) · Published by Druglandscape Ltd.