Article: The perils of percentages. (examples of statistical confusion resulting from government and business reports of percentage changes)(Brief Article)

AN AMUSING anecdote sent in by a reader provides the third theme in our occasional series on statistical abuse. In the late 1970s, the Mexico city government increased the capacity of the Viaducto, a four-lane motorway, by re-painting lines to make it six lanes wide-in effect, a 50% increase in capacity. Unfortunately, this also resulted in an increase in fatal accidents. After a year the Viaducto was changed back to a four-lane road-a 33% capacity reduction. The government, casting around for facts to support its claims of social progress, subtracted that 33% reduction from the 50% increase to claim a net increase in capacity of 17%.

This example demonstrates the ...

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