What you'll learn
How to measure how spread out a data set is.
Key measures
- Range = highest − lowest. Simple but affected by outliers.
- Quartiles: the lower quartile (Q1), median (Q2) and upper quartile (Q3) divide ordered data into quarters.
- Interquartile range (IQR) = Q3 − Q1 — the spread of the middle 50%, unaffected by outliers.
- Standard deviation: a measure using all values that shows spread about the mean.
Outliers
- A common rule: values more than 1.5 × IQR beyond Q1 or Q3 are outliers.
Exam tips
- Order the data before finding quartiles.
- Use the IQR (not the range) to compare consistency between data sets.
Common mistakes
- Mixing up Q1 and Q3.
- Forgetting to order the data first.