Year	Percent
1960	29.36
1961	29.11
1962	29.14
1963	28.49
1964	27.5
1965	26.53
1966	25.26
1967	24.49
1968	23.92
1969	23.48
1970	22.7
1971	22.37
1972	21.87
1973	19.37
1974	19.35
1975	20.29
1976	20.61
1977	19.23
1978	19.01
1979	18.5
1980	18.24
1981	17.6
1982	19.34
1983	17.9
1984	17.87
1985	17.99
1986	17.86
1987	17.01
1988	17.37
1989	18.03
1990	18.05
1991	17.91
1992	18.05
1993	18.17
1994	17.89
1995	17.2
1996	17.82
1997	16.9
1998	15.85
1999	15.87
2000	16.27
2001	16.16
2002	16.24
2003	16.29
2004	15.65
2005	15.34
2006	15.17
2007	15.68
2008	16.3
2009	16.83
2010	15.81