Year	Percent
1960	19.68
1961	19.06
1962	20.1
1963	21.21
1964	20.48
1965	20.61
1966	21.98
1967	22.33
1968	23
1969	24.65
1970	26.13
1971	27.53
1972	28.2
1973	27.74
1974	28.15
1975	29.89
1976	29.07
1977	27.34
1978	29.64
1979	28.12
1980	28.33
1981	28.46
1982	28.79
1983	29.39
1984	29.92
1985	29.56
1986	29.55
1987	28.59
1988	27.34
1989	26.9
1990	22.67
1991	24.05
1992	22.95
1993	22.89
1994	20.98
1995	21.84
1996	22.1
1997	21.28
1998	21.82
1999	22.15
2000	21.32
2001	21.86
2002	21.12
2003	21.52
2004	22.15
2005	22.51
2006	21.43
2007	20.33
2008	20.61
2009	22.84
2010	24.19