I suppose that there is some speacial agency in charge of statistics and they have found out it using different marks: a number of passangers , arrived in airports of Paris, a number of tickets sold in museums etc., a number of places booked in hotels. It is just my opinion, but it is really interesting to know how excatly:) 

..

@ Raff i lived there , i'm definitely agree with you,

Maybe, I don't reccommend, live, just vacation, a time ^^

I agree with you Andrey, statistics is my field and of course its difficult to know how exactly.

@Didi, i mean just in formal way ;) i have no idea about Afganistan.