Year	Percent
1971	32.21
1972	28.32
1973	25.5
1974	26.28
1975	25.92
1976	24.52
1977	24.32
1978	21.11
1979	22.24
1980	23.75
1981	23.33
1982	21.42
1983	19.74
1984	21.01
1985	19.17
1986	15.84
1987	16.43
1988	18.14
1989	19.55
1990	20.49
1991	21.41
1992	23.52
1993	22.26
1994	22.56
1995	21.26
1996	25.08
1997	25.63
1998	24.34
1999	24.77
2000	24.98
2001	24.19
2002	24.62
2003	25.53
2004	24.15
2005	23.37
2006	23.32
2007	22.32
2008	22.63
2009	22.76
2010	22.15