Line graphs, the mean and pie charts · Usually met around P6
Choosing charts, and charts that mislead
Compare: bars; change: a line; shares: a pie.
How to do it
Method
Compare: bars; change: a line; shares: a pie
Scale from 40: 70 looks 3 times as tall as 50
Another way
Read the real numbers
70 − 50 = 20, 70 ÷ 50 = 1.4
In full
Choose a chart for what you want to show: to compare how many of each, a bar chart (or a pictogram for small, simple numbers); to see how something changes over time, a line graph; to see each part's share of the whole, a pie chart. Watch out for charts that mislead: if a bar chart's scale doesn't start at 0, the difference in the bars is blown up — starting at 40, a bar for 70 looks 3 times as tall as one for 50 (30 against 10), yet 70 is only 20 more than 50, 1.4 times as much. A pie chart's parts always add to 100% (360°); a pictogram's pictures must all be the same size. Check by reading the real numbers and comparing those, not the size of the drawing.
Worked examples
Have a go first, then open each one for the answer and the working.
Example 1
This chart's scale doesn't start at 0. A's bar looks □ times as tall as B's; in fact A is only □ more than B.
Fill in each box
Show the answer and working
Answer: 4，15
Measured from 30, the bars stand 15 + 5 against 5; the real numbers are 50 and 35, only 15 apart.
Example 2
This bar chart's scale starts at 0.
True or false?
Show the answer and working
Answer: True
Look at the lowest number on the scale: if it isn't 0, the difference in the bars is blown up.
Example 3
Ka-yee's family spends $14,000 a month and wants to show what share goes on food, transport, rent and the rest. Which chart suits it best?
- A. Pictogram
- B. Bar chart
- C. Line graph
- D. Pie chart
Choose the right answer
Show the answer and working
Answer: D. Pie chart
A pie chart shows each part's share of the whole.
Practise it in the game →Line graphs, the mean and pie charts · Free to start, unlimited questions, no download