| | Jefferson, IA | Gillette, WY | United States |
| Population | 4,178 | 33,047 | 329,725,481 |
| Female Population | 54.0% | 48.2% | 50.5% |
| Male Population | 46.1% | 51.8% | 49.5% |
| Median Age | 47.7 | 32.3 | 38.4 |
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| Population - 2010 | 4,355 | 31,558 | 309,327,143 |
| Population - 2000 | 4,626 | 19,646 | 285,036,114 |
| Population - 1990 | 4,292 | 17,635 | 251,960,433 |
| Pop. 2000 to Now | -11.9% | 64.0% | 15.6% |
| Pop. 1990 to Now | -5.1% | 82.7% | 30.8% |
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| Population Density | 700.1 | 1,437.7 | 93.4 |
| Land Area | 6.0 | 23.0 | 3,531,905.4 |
| Water Area | 0.0 | 0.0 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Jefferson, IA | Gillette, WY | United States |
| Age 0 to 4 | 5.2% | 8.2% | 5.9% |
| Age 5 to 9 | 4.3% | 9.9% | 6.1% |
| Age 10 to 14 | 7.1% | 8.4% | 6.6% |
| Age 15 to 17 | 3.8% | 4.0% | 3.9% |
| Age 18 to 20 | 2.5% | 3.6% | 4.0% |
| Age 21 to 24 | 5.2% | 3.9% | 5.2% |
| Age 25 to 34 | 8.6% | 16.4% | 13.8% |
| Age 35 to 44 | 11.4% | 14.0% | 12.9% |
| Age 45 to 54 | 8.4% | 8.6% | 12.6% |
| Age 55 to 59 | 6.3% | 7.3% | 6.7% |
| Age 60 to 64 | 8.8% | 5.9% | 6.3% |
| Age 65 to 74 | 14.3% | 6.8% | 9.6% |
| Age 75 to 84 | 8.7% | 2.4% | 4.5% |
| Age 85 and over | 5.5% | 0.7% | 1.9% |
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| Race | | | |
| | Jefferson, IA | Gillette, WY | United States |
| White | 92.8% | 84.9% | 59.4% |
| Black | 0.1% | 0.4% | 12.2% |
| Asian | 0.4% | 0.1% | 5.6% |
| Native American | 0.0% | 0.8% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.1% | 0.2% |
| Other | 0.0% | 0.0% | 0.4% |
| Two or More Races | 2.4% | 3.8% | 3.2% |
| Hispanic | 4.3% | 10.1% | 18.4% |
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| Marriage & Family | | | |
| | Jefferson, IA | Gillette, WY | United States |
| Married Population | 45.6% | 58.9% | 49.8% |
| Currently Married | 43.7% | 56.7% | 48.0% |
| Married but Separated | 1.9% | 2.2% | 1.8% |
| Single Population | 54.4% | 41.1% | 50.2% |
| Never Married | 24.1% | 25.7% | 33.8% |
| Divorced | 16.0% | 11.6% | 10.8% |
| Widowed | 14.4% | 3.8% | 5.6% |
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| Household Size | 2.07 | 2.73 | 2.60 |
| Households | 1,965 | 11,954 | 124,010,992 |
| Family Households | 995 | 8,301 | 80,755,759 |
| Married couple, w/children | 22.8% | 39.1% | 30.5% |
| Married couple, no children | 52.1% | 38.4% | 42.9% |
| Non Family Households | 970 | 3,653 | 43,255,233 |
| Single householder, w/children | 12.4% | 19.9% | 15.8% |
| Single householder, no children | 12.8% | 2.7% | 10.8% |