It is probably referring to the fact that income for the bottom 50% of US earners has remained largely stagnant since the 1970s (after accounting for inflation)
Consider a nation with Steve (income $50) Bill (income $50) and George (income $100). The median is $50. Now suppose Steve's income remains the same, Bill's goes up to $60, and George's income goes up to $150. The median has gone up to $60, right?
However, suppose during this time, Jose (income $30) and Hector (income $40) immigrated. The median is back down to $50 even though every single person saw their income go up.
The longitudinal data I link to demonstrates that this is exactly what happened (albeit with different numbers).
(Note: this is an argument against drawing conclusions from Simpson's paradox, not an argument against immigration.)
http://www.chrisstucchio.com/blog/2011/immigrants_simpsons_p...
Consider a nation with Steve (income $50) Bill (income $50) and George (income $100). The median is $50. Now suppose Steve's income remains the same, Bill's goes up to $60, and George's income goes up to $150. The median has gone up to $60, right?
However, suppose during this time, Jose (income $30) and Hector (income $40) immigrated. The median is back down to $50 even though every single person saw their income go up.
The longitudinal data I link to demonstrates that this is exactly what happened (albeit with different numbers).
(Note: this is an argument against drawing conclusions from Simpson's paradox, not an argument against immigration.)