Quite right, trainer. Big Mac used to be an absolutely standardized product all over the world (I say this with all the authority of someone who has never been into a McDonald's in his life). Therefore economists found Big Mac pricing a good rough-and-ready guide to the cost/standard of living in different countries. Also, on Monday I heard a guy from the Carlsberg Brewery in Nottingham talking about the Beer Index saying that beer sales were a very good indicator of people's spending power - which, he claimed, was slowly picking up as UK started clawing its way out of the depth of the recession. Efes as a standardized mass product is actually quite a good indicator but heavily influenced by AKP deterrent pricing - for health reasons of course, not religious ones).
