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@Edison Chen said PPP is only to compare the living standards of people in different countries. For all other things we have nominal. Economists knew that the living standards of a nation depended upon its productivity. So they used gross productivity, i.e GDP, to compare the living standards between countries. But then some smart guy said, 'hey, wait a min, people in low income (GDP) countries are not having it that bad. They have lesser money but their things are also cheaper. So they are buying as much as higher income countries even with lower incomes.' So then the economists came up with PPP. If a person in rich country pays a doctor $50 and a person in poor country pays a doctor $5, we say both have the same purchasing power because both were able to pay for a doctor. In PPP calculation we take the money paid by the poor country guy as $50 instead of $5 because it bought the same service the as the rich country guy.
Now people will say that the poor country doctor must not be as good as the rich country doctor but the gap is not as big as people expect. A friend of mine lived many years in Kenya. He says that doctors in Kenya are much better than Pakistani doctors even though Kenya is poorer than Pakistan. This proves that the cheaper things people are buying in poorer countries are not necessarily of lower quality. So PPP serves alright in measuring the living standards.
But for measurement of status in global economy and international power we have nominal ofcourse.