How much political power can one number exert? Gross domestic product (GDP) is a strong candidate for the world's most potent numerical indicator. Politicians use it to rank states in order of production, and to guide policies to maintain their place in the pecking order. Its year-on-year changes dictate whether an economy is "in recession", which in turn influences what you pay for the loan to buy your home or run your business - and, indeed, the price of fish.
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