Year	Percent
1960	27.68869
1961	23.9839
1962	28.4448
1963	33.48998
1964	36.24664
1965	34.89572
1966	27.64319
1967	20.10985
1968	16.61525
1969	16.3554
1970	20.40596
1971	24.34317
1972	23.41912
1973	22.88335
1974	24.1829
1975	22.94662
1976	25.38159
1977	26.52148
1978	28.7298
1979	32.63526
1980	34.85333
1981	34.93934
1982	30.98339
1983	26.66961
1984	24.41792
1985	21.50017
1986	23.43249
1987	26.06784
1988	28.35203
1989	26.52351
1990	27.17052
1991	26.92142
1992	28.17687
1993	27.24987
1994	31.37927
1995	34.2907
1996	31.80035
1997	34.20019
1998	29.00386
1999	24.97951
2000	27.58241
2001	25.4912
2002	23.18095
2003	22.37964
2004	22.35743
2005	21.05982
2006	22.28515
2007	21.39608
2008	21.04121
2009	21.84686
2010	23.89047
2011	24.14318
2012	25.22013
2013	24.03039
2014	23.82247
2015	21.54148
2016	21.53775
2017	22.25054