I'm a little baffled about this: Let's say I want to compare the average income of 2 countries, A and B. Of course I'm interested in the real income so I adjust to PPP using the big mac index of that given year (dividing the average salary by some constant). Now I do it again, but 10 years later (with the big mac index of t = 10).
Question - Can I compare the adjusted average salary of country A, t=0 to the adjusted average salary of country A, t=10?
One can argue that no, because the index used for PPP adjustment changed over time, so we need to take account for this. That's why when you adjust for inflation you have specify to which year the prices have been fixed. You can't compare a real salary adjusted to 2003 prices to a real salary adjusted to 2013 prices.
My logic says yes, because assuming that McDonalds hasn't changed their Big Mac over the past 10 years, the unit of choice remained that same - one delicious burger and it's NOT like adjusting to inflation (in which you have to randomly assign a base year).