1. Draw a cumulative frequency table.
The table shows the times taken to complete 20 telephone calls.
Time
Cumulative Frequency
t  3
t  6
t  9
t  15
Time
Frequency
0  t  3 5
3  t  6 8
6  t  9 4
9  t  15 3 5 + 8 13 + 4 17 + 3 20
Draw a cumulative frequency table.

2. Drawing a cumulative frequency curve.
Time CF
t  3 5
t  6 13
t  9 17
t  15 20 21 18 15
Cumulative frequency 12 9 6 3 2 4 6 8 10 12 14 16
Time (mins)

3. Drawing a cumulative frequency curve.
Time CF
t  3 5
t  6 13
t  9 17
t  15 20 2 4 6 8 12 14 16 10 3 9 15 18 21
Time (mins)
Cumulative frequency

4. Finding the median, lower and upper quartile.2 4 6 8 12 14 16 10 3 9 15 18 21
Time (mins)
Cumulative frequency
The upper quartile is at the 75% mark.
The lower quartile is at the 25% mark.
The median is at the 50% mark.
Upper Q = 7
75% of 20 = 15
Median = 5
10 is 50% of 20
5 is 25% of 20
Lower Q = 3

5. 7 – 3 = 4 Finding the inter-quartile range 2 4 6 8 12 14 16 10 3 9 153 9 15 18 21
Time (mins)
Cumulative frequency
7 – 3 = 4
Upper Q = 7
Median = 5
The inter-quartile range is the difference between the upper and lower quartiles.
Lower Q = 3