| | Decorah, IA | Brooklyn, NY | United States |
| Population | 7,767 | 2,712,360 | 329,725,481 |
| Female Population | 52.9% | 52.4% | 50.5% |
| Male Population | 47.1% | 47.6% | 49.5% |
| Median Age | 34.1 | 35.7 | 38.4 |
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| Population - 2010 | 8,134 | 2,509,954 | 309,327,143 |
| Population - 2000 | 8,172 | 2,465,326 | 285,036,114 |
| Population - 1990 | 8,063 | 2,300,664 | 251,960,433 |
| Pop. 2000 to Now | -8.4% | 3.0% | 15.6% |
| Pop. 1990 to Now | -7.2% | 10.4% | 30.8% |
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| Population Density | 1,094.4 | 38,301.5 | 93.4 |
| Land Area | 7.1 | 70.8 | 3,531,905.4 |
| Water Area | 0.0 | 26.1 | 264,836.8 |
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| Est. Total Population By Age | | | |
| | Decorah, IA | Brooklyn, NY | United States |
| Age 0 to 4 | 4.7% | 7.1% | 5.9% |
| Age 5 to 9 | 2.8% | 6.2% | 6.1% |
| Age 10 to 14 | 4.0% | 6.2% | 6.6% |
| Age 15 to 17 | 2.6% | 3.5% | 3.9% |
| Age 18 to 20 | 16.6% | 2.9% | 4.0% |
| Age 21 to 24 | 11.7% | 4.9% | 5.2% |
| Age 25 to 34 | 8.6% | 18.0% | 13.8% |
| Age 35 to 44 | 8.7% | 14.2% | 12.9% |
| Age 45 to 54 | 7.7% | 11.7% | 12.6% |
| Age 55 to 59 | 5.5% | 5.8% | 6.7% |
| Age 60 to 64 | 4.0% | 5.4% | 6.3% |
| Age 65 to 74 | 12.3% | 8.2% | 9.6% |
| Age 75 to 84 | 6.8% | 4.1% | 4.5% |
| Age 85 and over | 3.9% | 1.8% | 1.9% |
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| Race | | | |
| | Decorah, IA | Brooklyn, NY | United States |
| White | 92.1% | 36.7% | 59.4% |
| Black | 1.2% | 28.6% | 12.2% |
| Asian | 1.6% | 11.7% | 5.6% |
| Native American | 0.0% | 0.1% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 0.0% | 0.7% | 0.4% |
| Two or More Races | 1.7% | 3.3% | 3.2% |
| Hispanic | 3.3% | 18.8% | 18.4% |
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| Marriage & Family | | | |
| | Decorah, IA | Brooklyn, NY | United States |
| Married Population | 38.5% | 43.1% | 49.8% |
| Currently Married | 37.9% | 40.3% | 48.0% |
| Married but Separated | 0.6% | 2.8% | 1.8% |
| Single Population | 61.5% | 56.9% | 50.2% |
| Never Married | 46.9% | 44.3% | 33.8% |
| Divorced | 7.8% | 7.4% | 10.8% |
| Widowed | 6.9% | 5.2% | 5.6% |
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| Household Size | 2.01 | 2.71 | 2.60 |
| Households | 2,865 | 985,108 | 124,010,992 |
| Family Households | 1,610 | 599,788 | 80,755,759 |
| Married couple, w/children | 28.8% | 28.9% | 30.5% |
| Married couple, no children | 51.1% | 33.2% | 42.9% |
| Non Family Households | 1,255 | 385,320 | 43,255,233 |
| Single householder, w/children | 9.9% | 18.8% | 15.8% |
| Single householder, no children | 10.3% | 19.1% | 10.8% |