| Year | 2nd generation | 1st generation |
|---|---|---|
| 1900 | 20.8 | 13.7 |
| 1910 | 20.1 | 14.6 |
| 1920 | 20.6 | 12.7 |
| 1930 | 20.1 | 11.3 |
| 1940 | 18.8 | 8.5 |
| 1950 | 16.2 | 6.9 |
| 1960 | 13.8 | 5.4 |
| 1970 | 12.1 | 4.7 |
| 1980 | 10.5 | 6.0 |
| 1990 | 9.7 | 7.9 |
| 2000 | 10.1 | 11.1 |
| 2006 | 10.8 | 12.2 |
| 2010 | 11.3 | 12.7 |
| 2015 | 11.9 | 13.9 |
| 2016 | 12.0 | 13.5 |
| 2017 | 12.0 | 13.7 |
- Report
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