Year	Percent
1971	31.69
1972	29.99
1973	30.43
1974	30.48
1975	27.54
1976	26.56
1977	26.5
1978	26.27
1979	26.74
1980	43.84
1981	44.9
1982	48.94
1983	46.39
1984	49.11
1985	50.16
1986	52.28
1987	45.25
1988	46.42
1989	41.99
1990	39.38
1991	38.26
1992	38.36
1993	36.28
1994	35
1995	32.94
1996	29.65
1997	29.92
1998	28.32
1999	26.99
2000	23.52
2001	22.93
2002	23.01
2003	22.62
2004	21.99
2005	22.38
2006	21.27
2007	20.85
2008	24.06
2009	23.6
2010	24.03