Year	Percent
1971	18.5
1972	18.79
1973	18.12
1974	18.55
1975	19.52
1976	20.4
1977	20.62
1978	19.99
1979	20.16
1980	20.37
1981	20.93
1982	21.56
1983	22.87
1984	23.53
1985	23.4
1986	24.34
1987	26.31
1988	26.02
1989	24.57
1990	20.62
1991	21.04
1992	20.67
1993	22.15
1994	21.34
1995	22.38
1996	21.69
1997	21.83
1998	22.15
1999	22.11
2000	22.17
2001	21.63
2002	22.08
2003	22.63
2004	22.32
2005	21.46
2006	22.05
2007	22.14
2008	22.15
2009	21.68
2010	21.42