Year	Percent
1976	45.30511
1977	47.25898
1978	48.40298
1979	46.48469
1980	51.92024
1981	51.59694
1982	50.66103
1983	50.67944
1984	48.84118
1985	45.42746
1986	43.39142
1987	43.65875
1988	44.81847
1989	45.94552
1990	46.05223
1991	47.04767
1992	47.53477
1993	48.43939
1994	50.39096
1995	51.03994
1996	51.55747
1997	52.61141
1998	52.56225
1999	54.47076
2000	54.28025
2001	54.69919
2002	55.57392
2003	55.40869
2004	55.91533
2005	58.08096
2006	61.84547
2007	61.89232
2008	61.61109
2009	62.23385
2010	62.85975
2011	63.19489
2012	63.61752
2013	64.37297
2014	65.67252
2015	66.29755
2016	66.99826
2017	66.92541