Year	Percent
1971	57.44
1972	57.3
1973	57.51
1974	57.06
1975	56.39
1976	56.03
1977	55.92
1978	56.2
1979	55.58
1980	56.08
1981	54.74
1982	54.09
1983	53.83
1984	52.25
1985	52.49
1986	51.49
1987	51.62
1988	50.27
1989	49.54
1990	48.87
1991	48.2
1992	47.83
1993	48.07
1994	47.07
1995	46.43
1996	46.41
1997	45.6
1998	46.49
1999	45.73
2000	45.4
2001	45.98
2002	45.14
2003	44.93
2004	44.33
2005	43.56
2006	42.26
2007	41.24
2008	40.08
2009	38.32
2010	37.65