Year	Percent
1970	59.94898
1971	60.34913
1972	60.82725
1973	61.28266
1974	61.71694
1975	62.35828
1976	62.74945
1977	63.20346
1978	63.55932
1979	63.97516
1980	64.5749
1981	64.9505
1982	65.31008
1983	65.8444
1984	66.04824
1985	66.54545
1986	66.90391
1987	67.24739
1988	67.69231
1989	68.0067
1990	68.3087
1991	68.59903
1992	68.87836
1993	69.30233
1994	69.66463
1995	69.91018
1996	70.29412
1997	70.62229
1998	70.98151
1999	71.32867
2000	71.76309
2001	72.08672
2002	72.53333
2003	72.83465
2004	73.25581
2005	73.66412
2006	74.18546
2007	74.5679
2008	74.96962
2009	75.4491
2010	75.91499
2011	76.39535
2012	76.74685
2013	77.28814
2014	77.72829
2015	78.15587