The percentage of women at the college, meanwhile, has nearly doubled, to 17 percent, since 1991.

The percentage of blacks who see progress has doubled in that period.

The percentage of Americans in prison has more than doubled since 1970 at a time when the crime rate was often falling.

The percentage of 30-year-olds with a bachelor's degree has more than doubled since 1970.

The percentage of voters with college degrees has doubled in the past 30 years.

Since 1999, the percentage of clean cars has more than doubled, the report said.

Four years later, the percentage of students passing the test had more than doubled.

From 1989 to 2001, the percentage of students treated for depression doubled.

Though, since 2002 the percentage of farmers under 35 has doubled from 3 to 6%.

The percentage of income tax paid by the average American has more than doubled since 1953.