This index (it is not official) is the basis for calculating purchasing power parity. The fact is that according to the economic PPP theory https://fbs.com/blog/what-is-the-big-mac-index-323 it is necessary to compare the currencies of different countries using the “basket of goods” approach. Under this concept, two currencies are in equilibrium, known as currencies at parity (at par), when a basket of goods is priced the same in both countries, adjusted for exchange rates.