Year	Percent
1971	30.11
1972	31.22
1973	32.47
1974	31.37
1975	33.79
1976	33.62
1977	33.14
1978	31.4
1979	30.81
1980	29.96
1981	30
1982	32.45
1983	30.59
1984	30.03
1985	30.44
1986	30.69
1987	31.39
1988	29.6
1989	28.05
1990	28.14
1991	27.85
1992	27
1993	27.65
1994	27.55
1995	26.99
1996	25.74
1997	24.96
1998	24.75
1999	24.2
2000	23.85
2001	24.74
2002	24.1
2003	23.39
2004	23.13
2005	22.42
2006	21.59
2007	22.2
2008	22.12
2009	22.65
2010	22.24