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Opens on Chapter 6’s example: the supplier’s tire brochure: 4,100 miles against 4,400, on bars drawn from 4,000.
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How it works
With an axis that starts at s, the bars you see are (a − s) and (b − s) tall, so the bigger one looks (b − s) ÷ (a − s) times the other, though it is only b ÷ a times as big. Tufte’s Lie Factor divides the change the picture shows by the change in the data, both as percents; an honest chart scores about 1.
From the book, Chapter 6. The brochure’s bars are 100 and 400 tall, so one looks 4 times the other; it is 1.073 times as big. The picture shows a 300% change for a real 7.3%: a Lie Factor of about 41. The bake sale’s bars look 5 times apart for a real 10.8% (Lie Factor 37). Every chapter and its calculator →
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