Year	Percent
1970	10.22
1975	13.5
1978	17.3
1979	17.58
1980	18.19
1981	18.3
1982	18.43
1983	18.69
1984	19.9
1985	20.82
1986	21.87
1987	22.22
1988	22.37
1989	21.64
1990	21.4
1991	21.4
1992	21.7
1993	22.4
1994	22.7
1995	23
1996	23.5
1997	23.7
1998	23.5
1999	23
2000	22.5
2001	22.3
2002	21.4
2003	21.6
2004	22.5
2005	23.8
2006	25.2
2007	26.8
2008	27.2
2009	27.8
2010	28.7
2011	29.5
2012	30.3
2013	30.1
2014	29.9
2015	29.3
2016	28.8
2017	28.11