Year	Percent
1971	21.13
1972	20.9
1973	20.01
1974	20.71
1975	20.31
1976	19.31
1977	18.16
1978	18.41
1979	18.77
1980	17.6
1981	16.34
1982	17.08
1983	17.82
1984	17.99
1985	18.65
1986	18.67
1987	19.01
1988	18.7
1989	19.57
1990	20.32
1991	21.06
1992	21.54
1993	23.46
1994	22.94
1995	21.63
1996	21.07
1997	20.81
1998	20.13
1999	20.74
2000	20.44
2001	23.55
2002	22.78
2003	23.02
2004	23.79
2005	24.25
2006	25.38
2007	24.66
2008	24.69
2009	24.5
2010	23.99