Year	Percent
1971	57.07
1972	56.18
1973	55.66
1974	54.91
1975	52.8
1976	51.88
1977	49.87
1978	48.22
1979	48.65
1980	48.8
1981	49.7
1982	48.91
1983	48.11
1984	46.59
1985	47.33
1986	46.21
1987	44.94
1988	44.18
1989	43.85
1990	44.22
1991	43.19
1992	41.86
1993	40.37
1994	38.69
1995	37.6
1996	37.03
1997	36.97
1998	35.03
1999	36.51
2000	36.72
2001	36.59
2002	35.44
2003	33.3
2004	30.21
2005	28.73
2006	27.32
2007	26.27
2008	24.89
2009	24.47
2010	23.47