Year	Percent
1980	56.59207
1981	57.30949
1982	57.44154
1983	57.11737
1984	55.67081
1985	55.08591
1986	55.50147
1987	55.4174
1988	55.63477
1989	55.90801
1990	56.84707
1991	57.29118
1992	59.21652
1993	59.43618
1994	58.62123
1995	57.64318
1996	59.03863
1997	59.20017
1998	59.12226
1999	59.69952
2000	59.85944
2001	60.15766
2002	60.46552
2003	60.76914
2004	60.50087
2005	60.86841
2006	60.35118
2007	60.05639
2008	61.05419
2009	62.88275
2010	61.05601
2011	61.81303
2012	63.25071
2013	64.13457
2014	64.58012
2015	65.68409
2016	65.56388
2017	65.18316