Year	Percent
1960	25.09
1961	24.26
1962	24.89
1963	26.1
1964	22.38
1965	19.93
1966	17.87
1967	16.52
1968	15.57
1969	13.99
1970	17.75
1971	16.46
1972	15.31
1973	15.05
1974	15.29
1975	16
1976	15.72
1977	15.92
1978	15.82
1979	16.28
1980	15.99
1981	16.75
1982	17.49
1983	18.59
1984	18.79
1985	27.92
1986	28.21
1987	27.46
1988	25.74
1989	25.3
1990	24.99
1991	26.32
1992	25.72
1993	25.54
1994	24.34
1995	23.45
1996	24.93
1997	23.72
1998	23.75
1999	24
2000	25
2001	25.28
2002	24.85
2003	25.29
2004	26.28
2005	25.94
2006	25.8
2007	24.27
2008	25.79
2009	27.16
2010	27.05