Year	Percent
1970	52.94817
1971	53.28226
1972	53.87595
1973	54.62259
1974	53.99045
1975	56.43127
1976	56.11497
1977	57.44548
1978	58.75086
1979	58.96011
1980	58.63308
1981	58.02595
1982	59.02685
1983	59.81099
1984	58.51863
1985	58.47284
1986	60.52317
1987	62.05137
1988	61.8277
1989	61.59559
1990	61.51085
1991	62.17468
1992	63.11685
1993	63.82368
1994	63.82785
1995	63.11378
1996	63.30903
1997	63.95039
1998	64.72548
1999	65.39348
2000	65.31499
2001	64.75553
2002	65.9117
2003	66.33831
2004	66.39723
2005	66.00404
2006	65.71808
2007	66.06477
2008	66.13274
2009	67.73003
2010	68.28833
2011	68.45499
2012	68.87009
2013	69.12668
2014	69.9717
2015	70.10946
2016	70.20797
2017	70.384