Year	Percent
1986	36.16
1987	35.53
1988	35.66
1989	35.17
1990	39.4
1991	37.95
1992	39.3
1993	39.7
1994	38.68
1995	36.09
1996	38.28
1997	38.19
1998	37.95
1999	36.06
2000	36.45
2001	36.73
2002	38.91
2003	41.74
2004	43.51
2005	43.92
2006	44.33
2007	45.25
2008	45.34
2012	48.2
2013	48.12
2014	47.98
2015	49.79
2016	51.12
2017	53.83