I'm not really sure how much you are familiar with Economist in general and the specifics of this survey, so bare with me if I misunderstood you.
First of all Economist is a highly credited business magazine that focuses both on specific topics, but also on the world of international business. Among its readers you will find top executives and diplomats from all around the world. Basically you can say thay anyone that does business in foreign countries read Economist. Part of the Economist is something named "Economist Intelligence Unit" (EIU) that provides the public, companies and governments worldwide strategic business information. EIU have published this cost of living index since 1946! The index is referenced in newspapers worldwide and both politicians and business decision makers pays attention to it. In fact, both large international companies and countries base their international travel expense plans and salary schemes to expats on the index.
So, this is no little insignificant index, but an index that carries a lot of weight internationally.