Year	Percent
1970	17.49
1971	17.89
1972	18.34
1973	19.99
1974	21.98
1975	23.76
1976	26.36
1977	27.53
1978	29.46
1979	30.14
1980	28.98
1981	27.75
1982	27.85
1983	29.11
1984	30.72
1985	30.8
1986	31.87
1987	34.03
1988	34.87
1989	35.16
1990	35.42
1991	35.95
1992	35.76
1993	34.02
1994	33.63
1995	33.44
1996	32.64
1997	31.43
1998	27.98
1999	27.51
2000	28.14
2001	27.48
2002	27.32
2003	27.61
2004	27.48
2005	26.85
2006	26.33
2007	25.92
2008	25.49
2009	24.5
2010	25.02
2011	24.85
2012	24.58
2013	24.52
2014	24.77
2015	25.2
2016	24.99
2017	25.07