Year	Percent
1960	18.3
1961	18.86
1962	19.16
1963	18.64
1964	18.22
1965	18.32
1966	18.14
1967	17.6
1968	17.44
1969	18.03
1970	18.92
1971	18.97
1972	19.06
1973	18.06
1974	17.95
1975	18.69
1976	18.54
1977	17.77
1978	17.61
1979	16.53
1980	16.44
1981	16.03
1982	16.8
1983	16.63
1984	16.36
1985	16.27
1986	16.27
1987	16.04
1988	16.31
1989	17.15
1990	16.23
1991	16.76
1992	16.85
1993	18.51
1994	17.89
1995	17.98
1996	18.43
1997	17.51
1998	16.95
1999	16.95
2000	17.08
2001	16.99
2002	17.08
2003	17.51
2004	16.88
2005	17.31
2006	16.21
2007	16.9
2008	17.41
2009	17.99
2010	17.88