Year	Percent
1970	9.01
1975	9.33
1978	12.18
1979	12.62
1980	13.06
1981	13.6
1982	13.45
1983	14.23
1984	16.06
1985	16.76
1986	17.18
1987	17.8
1988	18.28
1989	18.31
1990	18.5
1991	18.9
1992	19.8
1993	21.2
1994	23
1995	24.8
1996	26
1997	26.4
1998	26.7
1999	26.9
2000	27.5
2001	27.7
2002	28.6
2003	29.3
2004	30.6
2005	31.4
2006	32.2
2007	32.4
2008	33.2
2009	34.1
2010	34.6
2011	35.7
2012	36.1
2013	38.5
2014	40.6
2015	42.4
2016	43.5
2017	44.91