Year	Percent
1970	32.13
1971	32.45
1972	31.28
1973	31.64
1974	31.32
1975	28.99
1976	26.96
1977	27.43
1978	27.95
1979	26.98
1980	26.18
1981	26.28
1982	25.11
1983	24.9
1984	24.71
1985	24.36
1986	24.58
1987	25.23
1988	24.84
1989	25.21
1990	24.51
1991	24
1992	24.01
1993	23.9
1994	23.17
1995	22.81
1996	21.69
1997	21.62
1998	21.92
1999	22.24
2000	22.29
2001	21.69
2002	20
2003	19.57
2004	19.41
2005	19.04
2006	19.47
2007	19.18
2008	18.11
2009	14.46
2010	13.68
2011	13.5