Year	Percent
1971	28.86
1972	22.97
1973	19.76
1974	17.8
1975	16.72
1976	16.88
1977	16.42
1978	15.69
1979	13.94
1980	13.41
1981	11.87
1982	9.532
1983	9.07
1984	9.379
1985	9.9
1986	10.63
1987	13.7
1988	15.37
1989	15.04
1990	15.63
1991	15.44
1992	15.44
1993	15.45
1994	14.77
1995	14.23
1996	14.7
1997	15.41
1998	16.78
1999	16.69
2000	16.67
2001	17.34
2002	15.52
2003	16.22
2004	15.67
2005	17.39
2006	17.66
2007	16.75
2008	15.18
2009	13.42
2010	13.84