Year	Percent
1981	30.74
1982	30.67
1983	31.5
1984	31.39
1985	32.26
1986	32.42
1987	32.9
1988	33.72
1989	32.86
1990	33.9
1991	31.3
1992	31.41
1993	34.24
1994	33.67
1996	42.11
1997	43.68
1998	39.71
1999	37.47
2000	35.24
2001	35.06
2002	35.91
2003	40.94
2004	43.26
2005	43.3
2006	48.21
2007	48.49
2008	49.26
2009	52.55
2010	53.83
2011	53.73
2012	54.27
2013	53.5
2014	51.63
2015	48
2016	49.16
2017	51.02