Year	Percent
1971	23.58
1972	24.16
1973	24.93
1974	24.11
1975	23.59
1976	22.91
1977	22.18
1978	22.76
1979	22.26
1980	20.5
1981	19.7
1982	20.56
1983	21.07
1984	20.61
1985	21.38
1986	22.52
1987	22.81
1988	22.59
1989	22.08
1990	21.66
1991	21.88
1992	21.07
1993	19.27
1994	22.5
1995	21.92
1996	21.05
1997	21.34
1998	22.25
1999	20.19
2000	21.37
2001	20.81
2002	20.32
2003	21.06
2004	22.41
2005	19.15
2006	18.82
2007	18.56
2008	19.92
2009	22.14
2010	22.37