Year	Percent
1970	52.71084
1971	53.23529
1972	53.44828
1973	53.93258
1974	54.3956
1975	54.69169
1976	55.2356
1977	55.64103
1978	56.14035
1979	56.61765
1980	56.9378
1981	57.37705
1982	57.89474
1983	58.52018
1984	58.77193
1985	59.22747
1986	59.87395
1987	60.16427
1988	60.76459
1989	61.34122
1990	61.77606
1991	62.19282
1992	62.89425
1993	63.45455
1994	63.99287
1995	64.51049
1996	65.00858
1997	65.54622
1998	66.0066
1999	66.50485
2000	67.2496
2001	67.70671
2002	68.30015
2003	68.87218
2004	69.42393
2005	69.95646
2006	70.61341
2007	71.0084
2008	71.62534
2009	72.12449
2010	72.73936
2011	73.20261
2012	73.77892
2013	74.33628
2014	74.87562
2015	75.3978