Year	Percent
1981	28.97
1982	29.18
1983	27.09
1984	26.07
1985	25.66
1986	26.23
1987	26.06
1988	25.39
1989	25.74
1990	25.85
1991	25.7
1992	25.1
1993	24.2
1994	23.62
1995	23.2
1996	22.87
1997	22.46
1998	23.24
1999	22.96
2000	22.6
2001	23.06
2002	22.8
2003	22.54
2004	22.45
2005	22.39
2006	22.02
2007	22.44
2008	22.22
2009	21.13
2010	19.59
2011	17.69
2012	16.54
2013	15.56
2014	14.98
2015	14.94
2016	15.24
2017	15.41