| | Quincy, IL | Jacksonville, FL | United States |
| Population | 39,687 | 937,690 | 329,725,481 |
| Female Population | 52.1% | 51.4% | 50.5% |
| Male Population | 47.9% | 48.6% | 49.5% |
| Median Age | 40 | 36.2 | 38.4 |
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| Population - 2010 | 40,770 | 823,146 | 309,327,143 |
| Population - 2000 | 40,366 | 735,617 | 285,036,114 |
| Population - 1990 | 39,681 | 635,115 | 251,960,433 |
| Pop. 2000 to Now | -1.9% | 25.1% | 15.6% |
| Pop. 1990 to Now | -0.2% | 45.0% | 30.8% |
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| Population Density | 2,525.9 | 1,254.0 | 93.4 |
| Land Area | 15.7 | 747.7 | 3,531,905.4 |
| Water Area | 0.0 | 126.9 | 264,836.8 |
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| Est. Total Population By Age | | | |
| | Quincy, IL | Jacksonville, FL | United States |
| Age 0 to 4 | 6.3% | 6.7% | 5.9% |
| Age 5 to 9 | 6.4% | 6.3% | 6.1% |
| Age 10 to 14 | 6.1% | 6.4% | 6.6% |
| Age 15 to 17 | 2.8% | 3.5% | 3.9% |
| Age 18 to 20 | 3.0% | 3.6% | 4.0% |
| Age 21 to 24 | 5.4% | 5.2% | 5.2% |
| Age 25 to 34 | 14.0% | 16.4% | 13.8% |
| Age 35 to 44 | 11.8% | 13.0% | 12.9% |
| Age 45 to 54 | 10.9% | 12.3% | 12.6% |
| Age 55 to 59 | 6.5% | 6.8% | 6.7% |
| Age 60 to 64 | 6.5% | 5.9% | 6.3% |
| Age 65 to 74 | 10.8% | 8.7% | 9.6% |
| Age 75 to 84 | 5.8% | 3.7% | 4.5% |
| Age 85 and over | 3.7% | 1.5% | 1.9% |
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| Race | | | |
| | Quincy, IL | Jacksonville, FL | United States |
| White | 87.4% | 49.2% | 59.4% |
| Black | 6.1% | 30.3% | 12.2% |
| Asian | 0.8% | 4.8% | 5.6% |
| Native American | 0.1% | 0.1% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.1% | 0.2% |
| Other | 0.0% | 0.6% | 0.4% |
| Two or More Races | 3.7% | 4.0% | 3.2% |
| Hispanic | 1.9% | 10.9% | 18.4% |
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| Marriage & Family | | | |
| | Quincy, IL | Jacksonville, FL | United States |
| Married Population | 48.1% | 44.5% | 49.8% |
| Currently Married | 46.4% | 42.4% | 48.0% |
| Married but Separated | 1.8% | 2.1% | 1.8% |
| Single Population | 51.9% | 55.5% | 50.2% |
| Never Married | 30.3% | 36.0% | 33.8% |
| Divorced | 13.9% | 14.0% | 10.8% |
| Widowed | 7.6% | 5.5% | 5.6% |
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| Household Size | 2.27 | 2.50 | 2.60 |
| Households | 17,016 | 366,780 | 124,010,992 |
| Family Households | 9,565 | 222,308 | 80,755,759 |
| Married couple, w/children | 25.8% | 26.9% | 30.5% |
| Married couple, no children | 46.1% | 39.2% | 42.9% |
| Non Family Households | 7,451 | 144,472 | 43,255,233 |
| Single householder, w/children | 18.5% | 20.5% | 15.8% |
| Single householder, no children | 9.6% | 13.3% | 10.8% |