Cumulative frequency: GCSE Maths revision
Cumulative frequency is a running total. Plot against the upper bound of each group, then join with a smooth curve. Median is at n/2, quartiles at n/4 and 3n/4.
IQR
Interquartile range = Q3 − Q1. It measures the spread of the middle 50% and ignores extreme outliers.
Try these questions
Cover the answers and have a go first. Worked solutions are underneath.
There are 80 values. At what cumulative frequency do you read the median?
Q1 is 12 and Q3 is 28. IQR?
Common mistakes
- Plotting at the midpoint instead of the upper bound.
- Using n/2 on a list of group labels rather than on the vertical cf axis.
Answers and working
Frequently asked questions
Is a cumulative frequency graph the same as a histogram?
No. Histograms use frequency density and bar area. CF is a running total plotted as a curve.
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