Year	Percent
1971	50.4
1972	50.2
1973	46.88
1974	46.02
1975	43.41
1976	41.44
1977	37.88
1978	34.49
1979	29.99
1980	27.74
1981	26.6
1982	29.51
1983	30.34
1984	27.9
1985	25.59
1986	25.08
1987	25.86
1988	25.14
1989	25.8
1990	24.2
1991	23.21
1992	23.93
1993	23.44
1994	23.1
1995	23.19
1996	21.29
1997	21.84
1998	19.99
1999	21.35
2000	19.61
2001	20.03
2002	18.68
2003	17.93
2004	19.9
2005	19.55
2006	20.51
2007	21.58
2008	20.9
2009	18.83
2010	19.55