Why didn't someone think of this before?
Walter Dean Burnham made adjustments for the non-citizen population until 1986. Those who study voter turnout are deeply indebted to his painstaking research to construct historical voter turnout rates. Unfortunately, he went on a writing hiatus starting in the mid-1980s. He thus missed the increase in the non-citizen and ineligible felon populations, and missed explaining the often-discussed decline in contemporary turnout rates. Recently, he published updated turnout numbers in the Journal of the Historical Society. I am certain that if he had continued with his calculations after the 1990 census, he would have likely realized the rapid growth of the non-citizen population was affecting turnout rates.