Bar Charts
What is a bar chart?
- A bar chart is a visual way to represent discrete data
- Discrete data is data that can be counted
- This can be numerical like shoe sizes in a class
- Or non-numerical (categorical) like colours of cars down a road
- Discrete data is data that can be counted
- The horizontal axis shows the different outcomes
- The vertical axis shows the frequency
- The heights of the bars show the frequency
- Bars should be separated by gaps
- Bars should have equal widths
What is a dual (comparative) bar chart?
- Dual (or comparative) bar charts compare two data sets on one bar chart
- The data sets measure the same variable, so use the same scale
- The bars are in pairs (side-by-side) for each outcome
- e.g. For comparing the shoe sizes of two year groups
What is a bar-line chart?
- A bar-line chart also shows two data sets on one chart
- However one data set is represented by a line, and the other by bars
- This allows two different variables to be shown, with a different scale for each
- e.g. For showing the monthly temperature (as a line) and the monthly rainfall (as bars) across the year for a location
- One scale would be in °C, and the other would be in mm
- There are also composite bar charts, which are covered in their own section below
What is a line chart?
- A line chart, or vertical line chart is a visual way to represent discrete data
- Line charts are used for numerical data (rather than categorical data)
- They are particularly useful when there are lots of different options to show
e.g. Results of a test where scores are given as percentages
- They are particularly useful when there are lots of different options to show
- They are different to line graphs, which are used for time-series graphs
- Line charts are used for numerical data (rather than categorical data)
- The vertical axis shows the frequency
- The horizontal axis shows the different outcomes
- You can easily identify the mode (most common value) using a line chart
- This will be the outcome with the highest (tallest/longest) line
- e.g. In the line chart above, 11 was the modal test score, with a frequency of 7
- You can quickly see how the data is spread using a line chart
- Lines may be crowded around a particular group of options
- This may help identify anomalies or outliers in the data
- e.g. In the line chart above you can see
- the majority of the test scores, out of 20, were between 7 and 12
- one pupil scored 19 out of 20, much higher than anyone else in the class
Examiner Tip
- If asked to draw a bar chart, find the largest frequency and select a scale which allows it to fit in the space provided
Worked example
Mr Barr teaches students in Year 7 and Year 8.
He records the number of pets that students in each year have.
His results are shown in the dual bar chart below.
(a)
Write down the modal number of pets for his Year 7 students.
The modal number (mode) is the number of pets that occurs the most
Visually, this will be the highest bar for Year 7s
The mode for Year 7 is 1 pet
(b)
How many Year 8 students does he teach?
Add up all the heights (frequencies) of the Year 8 bars
4 + 8 + 4 + 3 + 0 + 2
He teaches 21 Year 8 students