Year	Percent
1971	17.97
1972	20.13
1973	20.6
1974	20.69
1975	21.71
1976	20.57
1977	21.88
1978	23.33
1979	21.8
1980	20.42
1981	19.67
1982	19.8
1983	19.02
1984	19.33
1985	17.93
1986	17.68
1987	17.24
1988	17.15
1989	15.34
1990	14.63
1991	13.66
1992	12.84
1993	12.36
1994	12.14
1995	11.35
1996	11.4
1997	10.81
1998	11.59
1999	11.39
2000	10.35
2001	10.16
2002	10.17
2003	9.957
2004	9.792
2005	9.973
2006	10.08
2007	9.25
2008	9.781
2009	9.886
2010	10.07