Data Handling
Cumulative Frequency:
The marks obtained by a group of students for maths are as follows:
| Marks (x) | frequency (f) |
| 0 - 20 | 3 |
| 21 - 40 | 6 |
| 41 - 60 | 9 |
| 61 - 80 | 8 |
| 81 - 100 | 4 |
Now, we can rearrange the table with a running-total as frequency. This frequency is called cumulative frequecny. A cumulative frequency table consists of classes
up to a certain upper value and cumulative frequencies, which takes the following form.
| Marks (x) | cumulative frequency (f) |
| up to 20 | 3 |
| up to 40 | 9 |
| up to 60 | 18 |
| up to 80 | 26 |
| up to 100 | 30 |
Now you can plot a graph to represent the above data and it looks like the following:
From the graph, we can find the the following:
- Lower Quartile - the class value for the 1/4th cumulative frequency = 36
- Median - the class value for the 1/2 of the cumulative frequency = 55
- Upper Quartile - the class value for the 3/4th cumulative frequency = 68
- Inter Quartile Range - the difference between the quartiles - 68 - 36 = 32
Now, we can draw a box-plot; it shows the minimum-value, LQ, Median, UQ and maximum value in that order as shown in the image.
If you would like to practise more, please visit this page.
Now, in order to complement what you have learnt so far, work out the following:
- The time taken for a certain test by a group of students are is as follows:
| Marks (x) | frequency (f) |
| 11 - 20 | 3 |
| 21 - 30 | 7 |
| 31 - 40 | 18 |
| 41 - 50 | 5 |
| 51 - 60 | 2 |
Find the following:
- the median
- quartiles
- IQR
- the pass mark, if 4/7th of the stuends are passed
- the number of students who scored more than 50
- The time taken by 25 kids to finish their lunch, in minutes, is as follows:
8, 9, 7, 11, 7, 12, 15, 9, 9, 10, 11, 17, 16, 10, 11, 6, 19, 12, 17, 15, 16, 12, 13, 13, 17
Construct an appropriate grouped-frequency table for this data and then plot a cumulative frequency graph for the same. Then calculate the
mean, median, quartiles and IQR for the data.