Pie Charts

Pie Charts

Pie charts

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

• Find the angles of the sectors by multiplying each fraction by 360°

  • Then check all angles add up to 360°

• 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)