Year	Percent
1960	0.25581
1961	0.31949
1962	0.39116
1963	0.42945
1964	0.41014
1965	0.35197
1966	0.28425
1967	0.24415
1968	0.24397
1969	0.30056
1970	0.39223
1971	0.49574
1972	0.58404
1973	0.63375
1974	0.64121
1975	0.60949
1976	0.57146
1977	0.54008
1978	0.50039
1979	0.4569
1980	0.40408
1981	0.3507
1982	0.30024
1983	0.26635
1984	0.26486
1985	0.28604
1986	0.31678
1987	0.34024
1988	0.35494
1989	0.35947
1990	0.35242
1991	0.34544
1992	0.34731
1993	0.34458
1994	0.35135
1995	0.35692
1996	0.37031
1997	0.37644
1998	0.37578
1999	0.3621
2000	0.34338
2001	0.31007
2002	0.29253
2003	0.2869
2004	0.30549
2005	0.33559
2006	0.3752
2007	0.40313
2008	0.42243
2009	0.41419
2010	0.39388
2011	0.36846
2012	0.35006
2013	0.32761
2014	0.3096
2015	0.29316
2016	0.27371
2017	0.25337