Year	Percent
1971	51.27
1972	49.2
1973	47.08
1974	47.63
1975	45.61
1976	44.76
1977	42.04
1978	40.23
1979	39.61
1980	40.41
1981	40.96
1982	45.89
1983	44.42
1984	46.05
1985	46.96
1986	46.92
1987	46.1
1988	45.01
1989	36.75
1990	36.24
1991	36.06
1992	32.69
1993	19.71
1994	19.23
1995	19.84
1996	20.21
1997	15.31
1998	14.11
1999	13.89
2000	14.25
2001	15.2
2002	16.02
2003	16.13
2004	17.82
2005	19.28
2006	20
2007	19.35
2008	19.78
2009	17.56
2010	18.78