| | Chelsea, MA | Central Islip, NY | United States |
| Population | 40,025 | 36,292 | 329,725,481 |
| Female Population | 48.8% | 48.8% | 50.5% |
| Male Population | 51.3% | 51.3% | 49.5% |
| Median Age | 33.6 | 34.7 | 38.4 |
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| Population - 2010 | 35,344 | 35,912 | 309,327,143 |
| Population - 2000 | 35,080 | 31,950 | 285,036,114 |
| Population - 1990 | 28,710 | 26,028 | 251,960,433 |
| Pop. 2000 to Now | 11.8% | 2.8% | 15.6% |
| Pop. 1990 to Now | 36.6% | 26.2% | 30.8% |
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| Population Density | 18,070.0 | 5,105.1 | 93.4 |
| Land Area | 2.2 | 7.1 | 3,531,905.4 |
| Water Area | 0.2 | 0.0 | 264,836.8 |
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| Est. Total Population By Age | | | |
| | Chelsea, MA | Central Islip, NY | United States |
| Age 0 to 4 | 8.5% | 7.0% | 5.9% |
| Age 5 to 9 | 5.5% | 4.7% | 6.1% |
| Age 10 to 14 | 7.2% | 7.7% | 6.6% |
| Age 15 to 17 | 4.1% | 4.9% | 3.9% |
| Age 18 to 20 | 3.5% | 6.1% | 4.0% |
| Age 21 to 24 | 4.9% | 5.4% | 5.2% |
| Age 25 to 34 | 18.9% | 14.7% | 13.8% |
| Age 35 to 44 | 15.8% | 14.9% | 12.9% |
| Age 45 to 54 | 11.7% | 14.0% | 12.6% |
| Age 55 to 59 | 5.3% | 5.9% | 6.7% |
| Age 60 to 64 | 5.1% | 5.0% | 6.3% |
| Age 65 to 74 | 5.4% | 5.8% | 9.6% |
| Age 75 to 84 | 2.5% | 3.4% | 4.5% |
| Age 85 and over | 1.7% | 0.5% | 1.9% |
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| Race | | | |
| | Chelsea, MA | Central Islip, NY | United States |
| White | 19.7% | 17.3% | 59.4% |
| Black | 7.0% | 20.9% | 12.2% |
| Asian | 3.5% | 2.0% | 5.6% |
| Native American | 0.0% | 0.0% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.4% | 0.7% | 0.4% |
| Two or More Races | 3.3% | 1.4% | 3.2% |
| Hispanic | 66.1% | 57.7% | 18.4% |
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| Marriage & Family | | | |
| | Chelsea, MA | Central Islip, NY | United States |
| Married Population | 41.5% | 36.9% | 49.8% |
| Currently Married | 36.9% | 34.3% | 48.0% |
| Married but Separated | 4.6% | 2.6% | 1.8% |
| Single Population | 58.5% | 63.1% | 50.2% |
| Never Married | 44.7% | 47.7% | 33.8% |
| Divorced | 8.4% | 12.3% | 10.8% |
| Widowed | 5.5% | 3.2% | 5.6% |
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| Household Size | 2.94 | 3.87 | 2.60 |
| Households | 13,353 | 9,331 | 124,010,992 |
| Family Households | 7,470 | 6,638 | 80,755,759 |
| Married couple, w/children | 34.0% | 29.4% | 30.5% |
| Married couple, no children | 26.6% | 26.4% | 42.9% |
| Non Family Households | 5,883 | 2,693 | 43,255,233 |
| Single householder, w/children | 25.3% | 22.2% | 15.8% |
| Single householder, no children | 14.1% | 22.0% | 10.8% |