[1][2] Successive updates have added countries (currently 183), years (1950-2019), and data on capital, productivity, employment and population.
By contrast, PWT uses detailed prices within each country for different expenditure categories, regardless of whether the output is traded internationally (say, computers) or not (say, haircuts).
The detailed prices used to compute PPPs are based on data published by the World Bank as part of the International Comparison Program (ICP).
The most common argument to explain this finding is the Balassa-Samuelson effect, which argues that as countries grow richer, productivity increases mostly in manufacturing and other traded activities.
The database gets its name from the original developers at the University of Pennsylvania, Robert Summers, Irving Kravis and Alan Heston.