In general, our results comparing salaries by region did not account for the costs of living. To make the comparisons more meaningful, we used purchasing power parity (PPP) numbers. The PPP essentially enables a salary comparison in different regions by asking how much a basket of goods would cost in the given country. Our data are based on the University of Pennsylvania's PPP data (see go.nature.com/fu7xcy; note that to control for widespread currency fluctuations in recent months, we used three different exchange rates from three different time points over the past few months).
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