An American, a Frenchman and a physicist were talking about some unusual weather. “It was twice as hot this afternoon as this morning”, said the American, “the temperature went up from 40 to 80 degrees.” The Frenchman interjected: “That’s in Fahrenheit. In Celsius, it was six times hotter.” The physicist was scornful. “On the only really scientific measure, the Kelvin scale, the increase was a piffling 5 percent.”
Who’s right? Well, all the measures are accurate and it certainly was hotter. But no single ratio – whether twice, six times or 5 percent – captures just how much hotter it actually felt. The feeling of hotness, like the feelings of pain or anger, cannot be measured with genuine precision.
It is the same for the feeling of prosperity – any measure will be arbitrary and quite possibly misleading. Consider gross domestic product, the most common index of economic success. GDP is the sum of spending on everything in the economy, from shoes to shoe-shines, from cars to child care. In comparing countries with each other or over time, GDP is usually adjusted for inflation to calculate what is ambitiously called “real GDP”. It is then often divided by the population, creating “real GDP per person”. This is usually measured in “constant dollars” and, for 2011 in the United States, becomes $43,149 of 2005 dollars.
Economists recognise that GDP is far from perfect. In 2009, a French government commission suggested that it should be augmented by measures of the distribution of wealth, environmental sustainability and “quality of life”. The Human Development Index, which is widely used by the United Nations, combines GDP with life expectancy and years of schooling.
These modifications are welcome, but they fail to correct GDP’s main weakness – that is what might be called the fallacy of precision. The human meaning of prosperity simply cannot be reduced to numbers. Supposedly exact measures generally confuse more than they illuminate.