Pie Charts (Cambridge O Level Maths)

What is a pie chart?

• A pie chart is a circle which is divided into slices (sectors) to show proportion
• They show the relative size of categories of data compared to each other, rather than their actual size or number
• For example if we were looking at the proportions of men and women working in a company, we are more interested in the relative sizes than the actual numbers of men and women
• There are 360° in a circle, and we can use this to help us calculate the size of each slice of the pie chart

How do I draw a pie chart?

• This is shown easiest through an example
• The following data is collected for a class of 30 students about their favourite colour

• STEP 1 – Find the number of degrees that represents 1 student
  There are 30 students in total, so 360° = 30 students
  Divide both sides by 30, so 12° = 1 student
• STEP 2 - Calculate the angle for each category by finding a fraction of 360°
  11 students out of 30 said red was their favourite colour, so this is
  4 students out of 30 said purple, so this is
  Repeat this for each category, they should sum to 360° in total

• STEP 3 – Draw the pie chart, using a protractor to measure the angles
  Start by drawing a vertical line from the centre of the circle to the top ("12 o'clock")
  Then use your protractor to measure the first angle, and draw a line to this point
  Move your protractor to this line, and repeat for each category
  You should include a key or labels to show which slice represents which category
  

How do I interpret a pie chart or find missing information?

• It is easy to spot from a pie chart which category is the largest or smallest proportion, but you may be asked to do something more advanced like finding some missing information
• Remember that all of the data is represented by 360°
• You can use this to find either how many degrees each person/piece of data is represented by, or how many people/pieces of data 1 degree represents
• For example if you are told that there is a slice measuring 30° which represents 15 people
  •  30° = 15 people
  • 1° = 0.5 people (by dividing by 30)
• 2° = 1 person (by dividing first statement by 15 or doubling the second statement)
  You can then use this information to help solve problems or find missing information