Reading media graphs critically — Mathematics, 11–13
The same data can look very different depending on the scale and the type of chart; check the axis, the source and whether the chart fits the story.
The same data can tell two stories
A graph is not the data, it is a way of showing it, and some ways mislead even when every number is true. Before you believe a graph, check: does it have a title, a source and a legend? What do the numbers on the axis say, does the scale start at 0 and are the steps equal? Is this the right kind of chart for the data? Comparing two graphs of the same data is the best way to find out.
Graphs are made to persuade
Someone who makes a graph for a newspaper, an advert or a website wants you to notice something, and the choices of scale, colour and chart type change what you notice. That is not always dishonest, but it means you have to read on purpose. When you make your own graph you have the same power, so you decide which chart shows the story honestly and you can explain your choice: bars to compare, a pie chart for a few parts of a whole.
Same numbers, two scales
A club had 40 members last year and 44 this year. Chart A has a scale from 0 to 50: the bars are 40 and 44 units tall, so the second is only a little taller (44 ÷ 40 = 1.1 times). Chart B starts its scale at 38: the bars are 40 − 38 = 2 and 44 − 38 = 6 units tall, so this year's bar looks 6 ÷ 2 = 3 times taller. The real increase is 44 − 40 = 4 members, 4/40 = 1/10, or 10%. Chart B does not lie in the numbers, but it exaggerates.
A pie chart is not always the best choice
A pie chart looks friendly, so many people use it for everything. But imagine a survey about favourite school subjects with eight subjects at 16%, 14%, 13%, 12%, 12%, 12%, 11% and 10%: the sectors are almost the same size and the colours are hard to tell apart. Nobody sees which is bigger, 12% or 13%. With bars in order of size, the ranking is clear at a glance. Choose the chart for the question, and be ready to explain your choice.
Newspapers, adverts and the water bill
Graphs appear in newspapers, adverts, websites and even in household bills, such as the chart of your family's water use month by month. Cut one out, or take a photo, and check it: is there a source? Does the scale start at 0? Would another kind of chart tell the story more honestly? Doing that with a graph you meet in real life is exactly what statistical literacy means.
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