Year	Percent
1960	23.39623
1961	24.48445
1962	25.23113
1963	25.61846
1964	25.75311
1965	25.52377
1966	24.94711
1967	24.17738
1968	23.38696
1969	22.82197
1970	22.56327
1971	22.58645
1972	22.82228
1973	23.21474
1974	23.70282
1975	24.23548
1976	24.81245
1977	25.43073
1978	26.05565
1979	26.63752
1980	26.77292
1981	26.59405
1982	26.32888
1983	26.01476
1984	25.69917
1985	25.40455
1986	25.15638
1987	24.93685
1988	24.69268
1989	24.35807
1990	23.91438
1991	23.35854
1992	22.7506
1993	22.18748
1994	21.7723
1995	21.30511
1996	20.83629
1997	20.51257
1998	20.28876
1999	20.1096
2000	19.93341
2001	19.75613
2002	19.57306
2003	19.34289
2004	19.02146
2005	19.15119
2006	19.36331
2007	19.48183
2008	19.52274
2009	19.50244
2010	19.44801
2011	19.36265
2012	19.27023
2013	19.22072
2014	19.27537
2015	19.46623
2016	19.80046
2017	20.26403