How much will a Big Mac cost you in Lima? Or Abu Dhabi? The answers can tell you a lot about “purchasing power parity (PPP)” – whether exchange rates mean that a product costs the same in different countries. A tool to make this theory more “digestible” was launched by The Economist in 1986. It allows comparison of several base currencies to others around the world. As they wrote this month: “A Big Mac currently costs $5.06 in America but just 10.75 lira ($2.75) in Turkey, implying that the lira is undervalued.”

Since McDonald’s restaurants aren’t so common in Africa, the market research firm Sagaci Research invented the supplementary “KFC index” to analyse PPP there.

MARS BARS

In 1932, a factory in Slough produced the world’s first Mars bar. Fifty years later, Financial Times writer Nico Colchester pointed out that the price of the confectionary in Britain was neatly correlated with the buying power of pound sterling. By measuring the cost of things in Mars Bars, Colechester noted how graduate salaries had improved slightly in 40 years. Meanwhile, train fares had become cheaper but roast beef dinners in pubs had gone up by more than 60%.

BAKED BEANS AND POPCORN