A student measures the boiling point of a saltwater solution. She takes six readings using a thermometer. Her results are: 102.3 °C, 102.4 °C, 102.3 °C, 107.1 °C, 102.4 °C, 102.3 °C. Describe how the student should process these results to obtain a reliable value for the boiling point, and explain the decisions she must make along the way.

AQA A-Level Physics (7408) — 3.1.2 Limitations of Physical Measurements · Describe · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

1. Identify 107.1 °C as an anomalous result.
2. Exclude the anomalous result from further calculation.
3. Calculate the mean of the remaining five readings.
4. Explain that repeated measurements reduce random error and improve reliability.
5. Note that the remaining results show good repeatability; the small spread indicates the mean is reliable.

Examiner tips

  • Use the word ‘anomalous’ or ‘outlier’ to show you recognise the error.
  • Show the mean calculation explicitly and include units.
  • Mention that the spread (range) is small to justify reliability.
  • Use the phrase ‘reduced random error’ to link to the concept of repeatability.

Common mistakes

  • Including the 107.1 °C reading in the mean.
  • Failing to state why the result is anomalous.
  • Not calculating the mean of the remaining readings.

Mark scheme (5 marks)

  1. Identify 107.1 °C as an anomalous result / outlier
  2. State that the anomalous result should be excluded / not included in the calculation of the mean
  3. State that the mean (average) of the remaining results should be calculated
  4. Explain that repeated measurements reduce the effect of random error / improve reliability
  5. Recognise the remaining results show good repeatability / the spread of the remaining results is small, suggesting the mean is a reliable value

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