Year	Percent
1980	24.02
1981	22.14
1982	20.94
1983	21.67
1984	20.66
1985	20.96
1986	22.91
1987	24.67
1988	23.96
1989	26.18
1990	25.86
1991	26.33
1992	26.17
1993	25.68
1994	26.11
1995	24.06
1996	23.31
1997	23.64
1998	23.03
1999	23.23
2000	22.25
2001	23.32
2002	22.5
2003	22.19
2004	22
2005	21.55
2006	21.71
2007	22.18
2008	21.56
2010	19.45
2011	19.15
2012	18.12
2013	16.45
2014	17.64
2015	19.12
2016	18.57
2017	18.33