| Statistic | Duncan | Iowa | National |
| Population | 102 | 3,155,070 | 328,239,523 |
| Population density (sq mi) | 130 | 57 | 91 |
| Median age | 41.3 | 38.0 | 37.7 |
| Male/Female ratio | 2.4:1 | 1.0:1 | 1.0:1 |
| Married (15yrs & older) | 27% | 60% | 55% |
| Families w/ Kids under 18 | 100% | 44% | 43% |
| Speak English | 88% | 92% | 79% |
| Speak Spanish | 12% | 4% | 13% |
| Race | Duncan | Iowa | National |
| White | 89.22% | 90.90% | 73.35% |
| Black | 0.00% | 3.32% | 12.63% |
| Asian | 0.00% | 2.14% | 5.22% |
| American Indian | 0.00% | 0.32% | 0.82% |
| Native Hawaiian | 0.00% | 0.08% | 0.18% |
| Mixed race | 10.78% | 1.97% | 3.06% |
| Other race | 0.00% | 1.27% | 4.75% |
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