The Economist [unpaywalled]: “Sorting countries into rich and poor can be difficult. Measures such as GDP are affected by population size (more people generally mean more output). But adjusting for population alone is not enough. Dollar income per person does not account for differences in prices between countries (a Big Mac, for example, will set you back more in some places than in others, even after converting into dollars). Nor does it account for productivity (overall output per hour worked). To get a fuller picture, The Economist therefore ranks countries by three measures: dollar income per person, income adjusted for local prices (known as purchasing-power parity, or PPP) and income per hour worked. See where each country ranks below..”