A box plot shows five numbers: the minimum, the lower quartile, the median, the upper quartile and the maximum. The box goes from the lower quartile to the upper quartile, with a line at the median. The whiskers reach the smallest and largest values.
The middle half of the data lies inside the box. The width of the box is the interquartile range, IQR = upper quartile − lower quartile. A wider box means the middle values are more spread out.
Worked example
A box plot has lower quartile 12 and upper quartile 30. Find the interquartile range.
- IQR = upper quartile − lower quartile.
- 30 − 12 = 18.
Answer:18
Key words
- quartile
- a value that cuts the ordered data into quarters
- interquartile range
- upper quartile minus lower quartile
- whisker
- a line from the box to an end value
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Common Core-referenced: 6.SP.B.4