Collecting, showing and reading data
Maths · Key Stage 3 · The School
Which chart
BAR CHART compares separate categories. LINE GRAPH shows change over continuous time. PIE CHART shows parts of one whole and needs the parts to add to everything. SCATTER GRAPH shows whether two measurements are related. Choosing wrongly is the commonest error — a pie chart of unrelated quantities means nothing at all.
Four numbers
MEAN is the total divided by how many. MEDIAN is the middle value once ordered. MODE is the most common. RANGE is largest minus smallest. They answer different questions: for house prices the mean is dragged up by a few mansions, so the median describes a typical house far better. Say which you used and why.
Misleading graphs
A vertical axis that does not start at zero exaggerates small differences. Unequal intervals distort a trend. A 3-D pie chart makes the front slice look bigger. None of these is necessarily dishonest, and all of them mislead — so check the axis before reading the shape, every time.
Two shops' sales: 102 and 108 units. Draw a bar chart with the axis from 0 to 120 and the bars look almost identical, which is the truth — a 6% difference. Now draw it with the axis from 100 to 110 and the second bar looks EIGHT TIMES taller than the first. Nothing was falsified; one number was changed on the axis. This is why the first thing to check on any chart is where the vertical scale begins.