Bar Charts & Pictograms
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
- The mode is the outcome with the highest bar
- You can also get dual bar charts to compare two data sets
- Bars are in pairs (side-by-side) for each outcome
What is a pictogram?
- A pictogram is an alternative to a bar chart
- It is used in the same situations
- There are no axes
- Frequency is represented by symbols
- A key shows the value of 1 symbol
- For example, 1 symbol represents a frequency of 2
- Half and quarter symbols are often used
- The pictogram above shows the shoe sizes of students in a class
- As 1 picture of a shoe represents 2 students
- Half a shoe represents 1 student
- The number of students with a shoe size of 7, is 3
- As 1 picture of a shoe represents 2 students
Exam Tip
- If asked to draw a bar chart, find the largest frequency and choose a scale which makes that fit in the space provided.
- If asked to draw a pictogram, pick a symbol that is easy to duplicate and draw half (or quarter) of.
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 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
