They often present a best practice method, and if data sources and resources are not available to implement that method, other less desirable measurement options are presented. The measurement decisions made by statisticians can have a significant impact on the end result. 3. Currency unit and price level  Statistics must be expressed in a common unit for comparison.
A simple way to achieve this is to convert individual currencies to a base currency using exchange rates. However, this type of conversion can distort comparisons because of the impact of such things as shortterm currency fluctuations on exchange rates. This is why a lot of international comparisons are now performed using purchasing power parity (PPP).