Year	Percent
1971	0.4604
1972	0.46103
1973	0.46866
1976	0.513
1977	0.50237
1978	0.49947
1979	0.50785
1980	0.54449
1981	0.53412
1982	0.52568
1986	0.58199
1987	0.57482
1988	0.57169
1989	0.58041
1990	0.59096
1991	0.6054
1992	0.61965
2001	0.65898
2009	0.68632
2011	0.69377
2012	0.70143
2016	0.74569