Cumulative frequency is a running total of the frequencies. Plot each running total at the upper end of its class and join the points with a smooth curve.
To estimate the median, go across from half the total frequency to the curve, then down. For 60 values, read across from 30. The lower quartile is at a quarter (15) and the upper quartile at three quarters (45). The interquartile range is upper quartile − lower quartile.
Worked example
A cumulative frequency graph shows 80 values. At which cumulative frequency do you read the upper quartile?
- The upper quartile is three quarters of the way through.
- ¾ × 80 = 60.
Answer:60
Key words
- cumulative frequency
- a running total
- quartile
- a value a quarter or three quarters through the data
- interquartile range
- upper quartile − lower quartile
Practise this
New questions every time.
Common Core-referenced: HSS-ID.A.1