Period | Average annual rate | Years to duplicate |
1959-2015 | 1.83% | 38 |
1959-1970 | 3.26% | 21 |
1971-1982 | 3.40% | 21 |
1983-2000 | 0.58% | 120 |
2001-2015 | 0.95% | 74 |