Year	Percent
1971	48.87
1972	48.19
1973	46.33
1974	47.75
1975	46.88
1976	47.52
1977	47.44
1978	46.87
1979	46.85
1980	48.52
1981	50.11
1982	50.23
1983	49.07
1984	52.1
1985	53.45
1986	53.63
1987	51.86
1988	50.09
1989	49.23
1990	46.06
1991	47.38
1992	45.2
1993	42.91
1994	41.77
1995	37.64
1996	36.22
1997	34.02
1998	34.26
1999	33.84
2000	33.94
2001	32.93
2002	32.42
2003	31.68
2004	30.71
2005	29.71
2006	29.29
2007	27.88
2008	28.02
2009	27.3
2010	25.71