Year	Percent
1971	35.58
1972	33.5
1973	30.69
1974	28.86
1975	27.65
1976	25.94
1977	24.75
1978	22.98
1979	21.95
1980	21.79
1981	22.49
1982	20.86
1983	20.12
1984	19.73
1985	18.21
1986	16.65
1987	16.94
1988	16.62
1989	16.19
1990	16.17
1991	16.2
1992	16.15
1993	15.15
1994	14.37
1995	13.96
1996	13.7
1997	13.33
1998	13.33
1999	13.37
2000	13.49
2001	13.2
2002	13.11
2003	13.07
2004	12.66
2005	12.71
2006	12.46
2007	11.87
2008	11.68
2009	12.17
2010	11.23