Year	Percent
1970	52.59988
1971	52.82042
1972	52.60756
1973	52.99869
1974	54.89834
1975	55.82933
1976	56.19048
1977	57.25586
1978	57.0945
1979	56.96517
1980	57.83075
1981	58.74487
1982	58.71868
1983	59.37431
1984	60.06732
1985	60.05762
1986	60.56266
1987	61.0671
1988	61.29721
1989	61.83703
1990	61.99492
1991	62.81926
1992	63.57876
1993	64.83615
1994	64.95046
1995	64.77382
1996	65.10781
1997	65.39705
1998	65.48706
1999	65.88955
2000	66.3111
2001	66.93096
2002	67.41362
2003	68.0582
2004	68.16538
2005	68.52741
2006	68.71738
2007	69.0075
2008	69.65403
2009	70.75142
2010	70.65197
2011	70.17969
2012	70.2795
2013	70.30369
2014	70.34611
2015	70.20872
2016	70.33799
2017	70.23631