Pie Charts (CIE IGCSE Maths: Core)

What is a pie chart?

• A pie chart is a circle divided into sectors which is used to present data
• The sectors represent different categories
  • They show the relative proportions of the categories
  • They do not show the actual frequencies of each category

How do I draw a pie chart?

• This is easiest explained through an example
• The following data shows the favourite colours of a class of students

• Write the frequencies as fractions of the total number of students, 30

Colour	Red	Purple	Blue	Green	Yellow	Orange
Students	11	4	9	3	2	1
Fractions

• Find the angles of the sectors by multiplying each fraction by 360° 
  
  • Then check all angles add up to 360°

Colour	Red	Purple	Blue	Green	Yellow	Orange
Students	11	4	9	3	2	1
Fractions
Angles	132°	48°	108°	36°	24°	12°

• Draw a vertical line from the circle's centre to the top, then use a protractor to mark off the first angle
  • Draw a line from the centre to this first mark
    • Then, from this line, mark off the next angle (and so on)

How do I solve problems with pie charts?

• Use the following facts
  • angles are proportional to the frequencies of each category
  • 360° represents the total frequency 
• For harder problems, it helps to work out
  • what frequency is represented by 1°
  • what angle is represented by 1 unit of frequency
• For example, if a sector of 30° represents 15 people, then
  • 1° = 0.5 people (dividing by 30)
  • 2° = 1 person (multiplying by 2)
• These relationships can then be scaled up or down accordingly
  
  • If 1° = 0.5 people
    • then 360° = 180 people (multiplying by 360)