Interpret Population, Sales and Climate Data
A chart of data is shown and the question is about it: is it growing or shrinking? when was the peak? between which two years did it rise most? The answer is not calculated, it is READ — a judgement about the shape of the data. The data comes from 26 different scenarios — population, house prices, temperatures, electricity use, library loans, revenue and costs, arrivals and departures, two students' marks, two teams' goals — because a chart you cannot place cannot be interpreted.
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Reading a chart shows up in maths, geography, economics and science, and is taken for granted in all of them. It is one of the few skills examined in four places and taught in none.
At medium level the Y axis is truncated: the bar looks like a rocket and the real rise is 3 %. It is the commonest manipulation in the press and in company slides, and the only way to learn to spot it is to have it done to you.
Not the same exercise with more points. Easy: READ a value off the picture. Medium: COMPARE two points — how much it changed, where it rose most. Hard: the answer is not drawn and you have to WORK IT OUT.
Two series you subtract (revenue and costs → profit; births and deaths → whether the town grows or shrinks). One series you take the mean or the median of, with the other measure always among the options because mixing them up is THE mistake of the topic. And two people with four marks each: who has the better average and who has been more consistent.
It is the classic mistake when reading a chart, and one question type is dedicated to exactly that: which stretch rises the most, which is almost never the highest point.
"Marta scores 9, 4, 5 and 6 across four terms; Ivan scores 6, 7, 6 and 7. Who has the better average? Ivan, 6.5 against 6, even though the best single mark of the year — the 9 — is hers. One high peak with three low marks does not make up for them, and that is exactly what the chart shows at a glance."
Prefer to learn without screens? Try this analogue version:
Cut a chart out of a newspaper or print one from a statistics office.
Before reading the accompanying text, answer in writing: up or down? where is the peak? which stretch changes fastest?
ALWAYS check the vertical axis: does it start at zero? If not, look again knowing that and see whether your first impression changes.
Compare your reading with the headline. They do not always agree, and that is the lesson.
If you like Read the Chart, these activities work similar skills:
If you want to calculate mean, median, mode and range rather than interpret shape.
Population pyramids and rates, the same ideas applied to real data.
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