[twoodfin]

I would define absolute income mobility in terms of some globally comparable good. Dollars, PPP adjusted or not, for example.

Relative mobility to me means mobility within some particular income distribution: Moving from, say, an average household income in the first quintile to an average household income in the fifth quintile.

I prefer absolute mobility as a measure of economic mobility for the same reason that any rational actor would prefer to win a lottery for the difference between the income of an average bottom 20% household and an average top 20% household in the U.S.[1] vs. Sweden[2]. According to OECD, that's $75K vs. $37K.

[1] http://www.oecdbetterlifeindex.org/countries/united-states/

[2] http://www.oecdbetterlifeindex.org/countries/sweden/