Year	Percent
1971	41.56
1972	41.55
1973	38.65
1974	37.32
1975	37.21
1976	35.69
1977	34.88
1978	34.12
1979	32.71
1980	32.76
1981	33.72
1982	33.45
1983	32.72
1984	32.73
1985	32.45
1986	32.03
1987	32.6
1988	36.8
1989	35.98
1990	35.33
1991	33.46
1992	32.94
1993	31.94
1994	31.45
1995	30.13
1996	30.06
1997	29.83
1998	28.83
1999	28.16
2000	27.83
2001	27.44
2002	27.42
2003	30.09
2004	25.59
2005	21.93
2006	23.68
2007	21.23
2008	25.25
2009	22.53
2010	23.17