Year	Percent
1971	16.48
1972	16.32
1973	16.04
1974	17.18
1975	17.39
1976	17.61
1977	16.63
1978	17.49
1979	16.46
1980	17.77
1981	17.66
1982	17.39
1983	19.99
1984	21.99
1985	22.24
1986	19.19
1987	18.85
1988	20.19
1989	19.6
1990	20.84
1991	21.3
1992	21.08
1993	21.45
1994	19.15
1995	18.93
1996	20.26
1997	17.41
1998	17.2
1999	18.8
2000	19.65
2001	19.76
2002	19.37
2003	18.83
2004	18.65
2005	19.66
2006	20.05
2007	21.4
2008	21.1
2009	22
2010	22.8