Bar Charts & Pictograms
What is a bar chart and what is it used for?
- A bar chart is a visual way to represent qualitative and/or discrete data
- e.g. colours of cars, shoe sizes, names of students
- Bar charts tend to be used when
- there are only a small number of possible outcomes
- the data is categorical (non-numerical)
- The vertical axis shows the frequency
- The scale should start at zero and increase in equal amounts
- The horizontal axis shows the different outcomes
- Bars are used for each outcome and the height is the frequency
- Each bar should have the same width
- There should be a gap between each bar
- You can easily identify the mode using a bar chart
- The mode is the most common outcome
- This will be the outcome with the highest bar
- e.g. In the bar chart above, 10 is the modal shoe size with a frequency of 11
- You can use a comparative bar chart to compare two (or more) data sets
- For each outcome you would have a bar for each data set to show the frequencies
- The bars for each data set for each outcome would be side-by-side with no gap
- You would use colours or shading and a key to clearly show which bars belong to which data set
What is a pictogram and what is it used for?
- A pictogram is an alternative to a bar chart
- It is a visual way to represent qualitative and/or discrete data
- Like bar charts, pictograms tend to be used for categorical (non-numerical) data
- There are no axes on a pictogram
- Frequency is represented by symbols
- It is essential a key is included in a pictogram to state the frequency shown by one symbol
- Half and quarter symbols are often used
- Pictograms should adhere to similar rules to bar charts to make them easy to read and interpret
- All symbols should be the same size/shape
- Symbols should be lined up so the highest frequency (the mode) and other features can be easily seen
- A pictogram for the shoe sizes of class 11A would look like this
Examiner Tip
- If asked to draw a bar chart with no frequency axis provided
- think about whether it would be sensible or not for the frequency axis to go up in 1's or higher
- going up in 2's often makes for a smaller, smarter diagram
- going up in 4's, possibly 5's and 10's may be suitable depending on the frequencies involved
- If asked to draw a pictogram, pick a symbol that is
- easy to duplicate so it can be drawn at the same size over and over again
- easy to draw half or a quarter of
Worked example
Mr Barr teaches students in both Year 7 and Year 8. He records the number of pets that students in each year have and uses a bar chart to represent this information.
a)
Write down the mode for the number of pets for the Year 7 students.
The mode for Year 7 is one pet as that has the highest (shaded) bar
b)
How many Year 8 students are there altogether?
Add up the heights (frequencies) of the year 8 (striped) bars.
4 + 8 + 4 + 3 + 0 + 2 = 21
21
