fubar said - The survey doesn't provide any evidence that when two people do the same job, their salary differs according to their passport.

You're right, the survey doesn't show that. I think QLers have misunderstood what the survey is showing. It's more a reflection of the jobs different nationalities do.

However, it is well know that it is sometimes (often?) the case here in Qatar that different nationalities doing the same job are paid differently.