It's based on purchasing power parity.

这是建立在购买力对等基础上的。

In 2008, the per capita GDP measured by purchasing power parity was 21% of the US.

“2008年，美国人均GDP中购买力占比为21%。

The one that we’re using in this case, is the purchasing power parity exchange rates.

我们在这种情况下使用的是购买力平价汇率。