A bar chart shows items represented as vertical or horizontal bars. The length of each bar shows the number of times the item occurs.
Example one
A survey of primary schools was carried out in January 1997. The number of pupils and all teachers, including the head teacher, was recorded for each primary school surveyed. The pupil to teacher ratio (PTR) was calculated for each school. The data was recorded in the table shown below.
| Pupil teacher ratio (PTR) |
Less than 16 |
16 |
17 |
18 |
19 |
20 |
21 |
22 |
23 |
24 |
25 |
26 |
27 |
Over 27 |
|---|
| Percentage of schools with this PTR |
2.0 |
1.2 |
1.9 |
2.9 |
4.0 |
6.5 |
8.0 |
11.2 |
13 |
14.3 |
12.8 |
10.3 |
6 |
5.9 |
|---|
A bar chart was produced to help the interpretation of the data.
The bar chart shows the data in a visual form.
For each PTR value, a vertical bar has been drawn. The top of the bar represents the corresponding percentage value from the table. For example, the bar for a PTR less than 16 has a height of 2 on the vertical axis, representing 2%, and the bar for a PTR of 23 is represented by 13% on the vertical axis.
It is often easier to see the pattern in data when it is displayed as a graph rather than in a table. This bar chart shows that most schools have a PTR between 22 and 26 and the most common PTR is 24.
Often in published documents containing data, bar charts rather than tables are provided and information must be obtained by using the charts.
Example two
Use the bar chart to find what percentage of schools have a PTR of 20 or less.
This question can be solved by locating the bars for PTRs of 20 or less, ie. the bars for less than 16, up to and including 20. Because the values must be read from the bar chart, some values are likely to be approximate as they cannot be read accurately.
2 + 1.2 + 1.9 + 3 + 4 + 6.5
= 18.6%
So, approximately 18.6% of schools have a PTR of 20 or less.
Worked examples
Example one
How many pupils entered employment in 1998?
Look at the key to find the appropriate shading for 1998.
Find 'employment' and the end of the appropriately shaded bar for 1998. Find the corresponding number on the horizontal ‘number of pupils’ axis: 100.
So, 100 pupils entered employment in 1998.
Example two
This is a composite bar chart which shows the marks achieved by 11 pupils in three tests. Each test covers a different topic from the programme of study. The test result for each topic is shaded and forms part of each bar as shown on the chart.
Which of the following statements are true?
- Pupil S had the lowest total marks for all three tests
- Pupil V had the highest mark in topic 1
- None of the pupils scored 200 or more marks
- Pupil W had the highest score on topic 2
Look at each statement in turn and find the information on the graph. Check whether or not the statement is true.
- The lowest total mark is found by looking at the tops of all the bars. Pupil S had 160 marks. Pupil P had the lowest overall mark (157). So this statement is not true.
- Pupil V had 60 marks in topic 1, while Pupil R had the highest mark (74) in topic 1 so this statement is not true.
- Pupils R, T and W scored 200 or more marks so this statement is not true.
- Scores on topic 2 can be found by looking at the height of the dark shaded sections of each of the bars or by finding the difference between value on the vertical axis at the top of the dark shaded section of the bar and the value on the vertical axis at the bottom of the dark shaded section of the bar. Pupils T and W scored highly on topic 2. Comparing these on the chart it can be seen that pupil W has the highest score so this statement is true.
The only statement that is true is statement four.