Year	Percent
1971	23.27
1972	22.25
1973	21.62
1974	19.75
1975	19.6
1976	18.95
1977	18.66
1978	17.48
1979	16.71
1980	16.93
1981	15.68
1982	16.03
1983	16.07
1984	16.49
1985	15.98
1986	16.81
1987	16.23
1988	16.46
1989	16.76
1990	18.72
1991	18.25
1992	19.02
1993	18.97
1994	18.96
1995	19.42
1996	18.58
1997	18.28
1998	18.41
1999	17.64
2000	18.43
2001	18.53
2002	18.15
2003	18.31
2004	17.46
2005	16.82
2006	16.23
2007	15.75
2008	15.55
2009	16.07
2010	15.73