1. Cumulative Frequency - used to find the median and the inter quartile range (IQR)

2. Add a 3rd column and do a running total of the frequency column
A group of 50 nurses were asked to estimate a minute. The results are shown in the table.
Time
(seconds)
Frequency
20 < t ≤ 30 2
30 < t ≤ 40
40 < t ≤ 50 9
50 < t ≤ 60 13
60 < t ≤ 70 17
70 < t ≤ 80 3
80 < t ≤ 90
90 < t ≤ 100
Cumulative
Frequency 2 4 13 26 43 46 48 50
Scale for x axis, on your graph, is the end points of the class intervals
We now need to show this information in a graph
Scale for y axis, should be suitable for your data
- as there are 50 nurses. Final value should always be equal to the number of pieces of data in the question.

3. Estimating a minute

4. Find the median and interquartile range
Find the middle, ½ way up (median) of the data, ¼ (lower quartile) way up and ¾ (upper quartile)
What now??
Upper quartile
Median
Lower quartile
66 secs
50 secs
60 secs

5. Results Median = 60 seconds Inter quartile range
= upper quartile - lower quartile
= 66 – 50
= 16 seconds

6. Let’s try some questions page 55