Year	Percent
1970	18.76
1971	19.56
1972	20.21
1973	20.47
1974	20.96
1975	21.97
1976	22.58
1977	22.58
1978	23.42
1979	23.88
1980	26.34
1981	26.89
1982	26.77
1983	26.51
1984	26.54
1985	26.56
1986	26.62
1987	26.63
1988	26.86
1989	26.99
1990	27.45
1991	30.31
1992	29.92
1993	28.19
1994	28.1
1995	28.72
1996	30.52
1997	28.83
1998	29.25
1999	28.92
2000	29
2001	29.73
2002	32.71
2003	33.18
2004	36.37
2005	37.25
2006	38.69
2007	39.06
2008	39.69
2009	40.89
2010	40.67
2011	42.18
2012	42.08
2013	42.46
2014	42.72
2015	45.96
2016	47.02
2017	47.1