Year	Percent
1971	20.7
1972	21.42
1973	22.55
1974	22.25
1975	21.85
1976	24.89
1977	22.4
1978	23.13
1979	23.18
1980	23.82
1981	23.05
1982	22.79
1983	25.4
1984	26.95
1985	34.88
1986	36.09
1987	35.25
1988	35.01
1989	39.04
1990	34.82
1991	36.36
1992	36.12
1993	36.24
1994	38.1
1995	37.52
1996	37.99
1997	36.27
1998	39.99
1999	38.58
2000	38.38
2001	39.65
2002	38.82
2003	38.54
2004	41.39
2005	42.64
2006	41.77
2007	41.92
2008	42.29
2009	24.04
2010	24.04