Year	Percent
1971	33.23
1972	32.84
1973	32.97
1974	33.31
1975	32.91
1976	34.53
1977	33.98
1978	34.39
1979	34.3
1980	34.4
1981	34.81
1982	34.11
1983	33.85
1984	33.55
1985	33.14
1986	33.01
1987	33.65
1988	33.76
1989	34.33
1990	34.56
1991	34.81
1992	35.01
1993	34.97
1994	35.16
1995	35.19
1996	33.46
1997	34.68
1998	37.47
1999	52.4
2000	64.97
2001	66
2002	64.9
2003	63.71
2004	64.26
2005	63.91
2006	61.93
2007	58.89
2008	59.28
2009	57.3
2010	55.52