If there's been a large rise in the proportion of the African-American population that is low skill your measures of social science benchmarks are going to be distorted. Low-income people don't vote very much anyway as much as hiring come people but it's not like they don't vote so obviously it makes a difference. I found the data on the dropout rate to be particularly interesting and this is a general problem.
Becky Pettit of the University of Washington and author of Invisible Men talks with EconTalk host Russ Roberts about the growth of the prison population in the United States in recent decades. Pettit describes the magnitude of the increase particularly among demographic groups. She then discusses the implications of this increase for interpreting social statistics. Because the prison population isn't included in the main government surveys used by social scientists, data drawn from those surveys can be misleading as to what is actually happening among demographic groups, particularly the African-American population.