Reading charts with a critical eye
Axes, framing and comparisons: what to check when looking at a chart.
A chart can make a complicated relationship visible at a glance. It can also make that relationship appear more dramatic or certain than the data supports. This does not always involve deliberate deception. A choice of scale alone can change the impression. A few calm questions help.
Look at the axes first
What quantity is being shown, and in which unit? Does the scale begin at zero or another value? A narrow range can make small differences look large. That is not always wrong, but it should be clear. Also check whether equal distances on the time axis represent equal periods.
Comparisons need a reference
Absolute figures and proportions answer different questions. A larger city may record more events without each resident being more likely to experience them. A percentage without its starting value can also say little. Look for the source, period and definition. Without these details, even an elegant graphic is difficult to interpret.
Try this
- Read the title and measured quantity separately.
- Check the unit, starting point and time period.
- Find the source and definition of the data.
A useful chart makes its assumptions visible.
A thought to take away
Charts do not have to be complicated to deserve careful reading. Taking a moment to understand the scale helps you judge the actual claim. Sometimes the crucial detail is not the steep line, but the small label beneath it.
