Year	Percent
1981	31.36
1982	31.89
1983	32.32
1984	32.75
1985	33.04
1986	34.58
1987	34.7
1988	35.19
1989	35.96
1990	35.81
1991	38.02
1992	39.78
1994	43.94
1995	45.33
1996	46.24
2000	50.27
2001	50.07
2002	51.83
2003	53.25
2004	53.35
2005	53.35
2006	54.21
2007	54.52
2008	54.11
2009	55.58
2010	56.6
2011	56.4
2012	56.98
2013	57.43
2014	57.87
2015	57.75
2016	57.75
2017	57.89