No it also looks at income across generations within quartiles, e.g. "But [incomes of] 93% of the children in the poorest households — those in the bottom 20% — surpassed their parents...Julia Isaacs’s study for the Pew Charitable Trusts looking at the late 1960’s up to 2002 finds that children raised in the poorest families made the largest gains as adults relative to children born into richer families."