A student measures the density of a liquid by recording its volume and mass. He takes five readings of the volume using a measuring cylinder. His results are: 48.0 cm³, 48.2 cm³, 48.1 cm³, 51.3 cm³, 48.1 cm³. Identify the anomalous result. Explain what the student should do with this result and why. Then describe how the student can improve the reliability of his final mean volume.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
The anomalous result is 51.3 cm³.
The student should exclude this value when calculating the mean volume, because it is far from the other four readings and would skew the mean, giving a value that does not represent the true volume.
To improve reliability, the student should repeat the volume measurements several more times (e.g. 10–15 repeats). More repeats reduce the influence of random error and give a more accurate, reliable mean.
The student should exclude this value when calculating the mean volume, because it is far from the other four readings and would skew the mean, giving a value that does not represent the true volume.
To improve reliability, the student should repeat the volume measurements several more times (e.g. 10–15 repeats). More repeats reduce the influence of random error and give a more accurate, reliable mean.
Examiner tips
- Identify the outlier first; examiners look for the specific value. Explain why it skews the mean – mention accuracy and representation of the true value. State that it should be discarded, not just ignored. Show that more repeats reduce random error and improve reliability.
Common mistakes
- Failing to recognise 51.3 cm³ as the outlier. Including the outlier in the mean. Not explaining how additional repeats reduce random error.
Mark scheme (5 marks)
- Identifies 51.3 cm³ as the anomalous result
- States the anomalous result should be excluded / not used when calculating the mean
- Explains that including it would skew / affect the mean, making it less accurate / not a true representation of the true value
- States that the student should repeat the measurements (more times / increase the number of repeats)
- Explains that more repeats allow a more reliable mean to be calculated (by reducing the effect of random error)
Key terms in this question
anomalous result · mean · reliability
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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