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Make a pie chart

By David Shaw, 26 February 2013

You will need at least three pies.

Round circular graphs are known as pie charts. That name gave us an idea – what if they were made from pies?

Safety: When dealing with food, use clean hands and clean equipment!

You will need

• About 10 people
• 3 different circular pies, about the same size. Open topped tarts of different colours are best.
• Paper
• Pens
• Protractor
• Knife

Sort the responses and tally them all up.

What to do

1. Write the name of each pie on a piece of paper. Make one copy of this survey sheet for every person.
2. Give everyone a pen and a survey sheet. Show them the pies and ask them to write down the pie they most want to eat.
3. Collect up and count all the survey sheets.
4. Divide 360° by the number of survey sheets. This will give you an angle.
5. Sort the survey sheets, making a pile for each pie.
6. Find the centre of each pie.
7. For each pie, count the survey sheets and then multiply by the angle calculated in step 4. Measure this angle around the centre of the pie and cut a slice accordingly.
   

   Calculate the angle you will need to cut out of each pie.

8. Take the three slices you have cut and put them together to make a new pie. The size of each slice will represent how many people wanted to eat that pie the most.

What’s happening?

Pie charts are graphs. They are often used to show how something limited is shared. For example, a pie chart is good for showing how many hours a day you spend doing different things, or what you spend all your money on.

Use a protractor to measure the pieces you’re cutting.

To make a pie chart, convert all the quantities you have into angles. You want two important things to happen. First, the proportions need to remain the same. If you spend twice as long reading as watching TV, you want the reading angle to be twice as large as the TV angle. Second, all the angles need to add up to 360°, or they won’t make a whole circle.

The angles at the centre of the circle show the relative sizes of the things you are graphing. Importantly, the areas of the pieces also show this information. If you change the length of the pieces, so some of them are segments of larger circles, the area behaves differently. If you make a pie piece twice as long, keeping the angle the same, it will have four times the area. Although it can look good to make some slices longer, it can make the graph misleading.

The finished pie chart.

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