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Averages tell you about the centre of a data set, but you also need to describe how spread out the data is. Edexcel frequently asks you to compare two distributions using an average and a measure of spread.
| Term | Definition |
|---|---|
| Range | Largest value − smallest value |
| Interquartile range (IQR) | Upper quartile (Q3) − Lower quartile (Q1); the range of the middle 50% of data |
| Lower quartile (Q1) | The value one-quarter of the way through the ordered data |
| Upper quartile (Q3) | The value three-quarters of the way through the ordered data |
| Outlier | A value that is unusually large or small compared to the rest |
Range = largest value − smallest value
Example: Data: 3, 5, 7, 8, 12, 15, 42 Range = 42 − 3 = 39
The range is simple but has a major weakness: it is affected by outliers (extreme values). Here, 42 makes the range large even though most values are clustered between 3 and 15.
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