| | Spencer, IA | Zion, IL | United States |
| Population | 11,292 | 24,660 | 329,725,481 |
| Female Population | 52.6% | 54.3% | 50.5% |
| Male Population | 47.5% | 45.7% | 49.5% |
| Median Age | 40.8 | 32.6 | 38.4 |
| space3 | | | |
| Population - 2010 | 11,225 | 24,416 | 309,327,143 |
| Population - 2000 | 11,317 | 22,866 | 285,036,114 |
| Population - 1990 | 11,066 | 19,775 | 251,960,433 |
| Pop. 2000 to Now | -2.9% | 2.0% | 15.6% |
| Pop. 1990 to Now | -0.7% | 17.9% | 30.8% |
| space4 | | | |
| Population Density | 1,033.3 | 2,519.2 | 93.4 |
| Land Area | 10.9 | 9.8 | 3,531,905.4 |
| Water Area | 0.2 | 0.0 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Spencer, IA | Zion, IL | United States |
| Age 0 to 4 | 5.1% | 5.8% | 5.9% |
| Age 5 to 9 | 6.4% | 7.3% | 6.1% |
| Age 10 to 14 | 7.5% | 8.9% | 6.6% |
| Age 15 to 17 | 4.1% | 6.6% | 3.9% |
| Age 18 to 20 | 3.4% | 4.8% | 4.0% |
| Age 21 to 24 | 4.8% | 5.8% | 5.2% |
| Age 25 to 34 | 10.8% | 13.4% | 13.8% |
| Age 35 to 44 | 11.7% | 11.9% | 12.9% |
| Age 45 to 54 | 11.3% | 13.6% | 12.6% |
| Age 55 to 59 | 5.8% | 5.1% | 6.7% |
| Age 60 to 64 | 8.0% | 5.3% | 6.3% |
| Age 65 to 74 | 10.9% | 7.2% | 9.6% |
| Age 75 to 84 | 7.0% | 3.3% | 4.5% |
| Age 85 and over | 3.3% | 1.1% | 1.9% |
| space1 | | | |
| Race | | | |
| | Spencer, IA | Zion, IL | United States |
| White | 91.2% | 29.2% | 59.4% |
| Black | 0.1% | 25.9% | 12.2% |
| Asian | 0.7% | 4.3% | 5.6% |
| Native American | 0.2% | 0.1% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.3% | 0.6% | 0.4% |
| Two or More Races | 2.7% | 2.8% | 3.2% |
| Hispanic | 4.7% | 37.0% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Spencer, IA | Zion, IL | United States |
| Married Population | 49.2% | 40.6% | 49.8% |
| Currently Married | 48.5% | 38.8% | 48.0% |
| Married but Separated | 0.7% | 1.8% | 1.8% |
| Single Population | 50.8% | 59.4% | 50.2% |
| Never Married | 30.0% | 42.9% | 33.8% |
| Divorced | 12.4% | 12.0% | 10.8% |
| Widowed | 8.3% | 4.4% | 5.6% |
| space5 | | | |
| Household Size | 2.19 | 2.85 | 2.60 |
| Households | 5,027 | 8,503 | 124,010,992 |
| Family Households | 2,857 | 5,573 | 80,755,759 |
| Married couple, w/children | 25.5% | 28.5% | 30.5% |
| Married couple, no children | 48.5% | 31.1% | 42.9% |
| Non Family Households | 2,170 | 2,930 | 43,255,233 |
| Single householder, w/children | 20.7% | 30.0% | 15.8% |
| Single householder, no children | 5.3% | 10.4% | 10.8% |