http://www.theatlantic.com/business/archive/2015/07/america-...
http://www.economist.com/news/united-states/21595437-america...
https://www.brookings.edu/blog/social-mobility-memos/2015/05...
http://www.epi.org/publication/usa-lags-peer-countries-mobil...
http://www.economist.com/news/briefing/21640316-children-ric...
https://www.washingtonpost.com/news/wonk/wp/2016/08/03/there...
http://www.epi.org/publication/usa-lags-peer-countries-mobil...
It's not measuring upward mobility, but mobility in general. Seems suspect.
http://www.statcan.gc.ca/pub/11-626-x/11-626-x2016059-eng.ht...
The rich stay rich, more than the poor stay poor (in Canada).
But the number is about [relative change in income across] the population - if most people are poor (which they are) high mobility across the population likely means poor people are moving up (or indeed getting much poorer - halving your income when you're already in the lower quartile would probably be as big a shift as doubling it - if not more).
You're right to ask for more detail, though.