Year	Percent
1971	41.14
1972	41.88
1973	41.26
1974	39.93
1975	40.06
1976	40.21
1977	41.96
1978	43.07
1979	43.14
1980	43.46
1981	42.39
1982	43.27
1983	46.31
1984	45.57
1985	45.3
1986	44.13
1987	42.57
1988	42.98
1989	44.54
1990	42.48
1991	43
1992	42.47
1993	39.63
1994	37.39
1995	35.52
1996	34.65
1997	34.5
1998	34.41
1999	31.85
2000	32.59
2001	34.5
2002	33.3
2003	33.23
2004	29.7
2005	30.15
2006	30.38
2007	28.34
2008	22.41
2009	24.37
2010	26.12