Year	Percent
1960	19.28762
1961	18.77425
1962	17.37809
1963	17.53723
1964	17.87246
1965	18.27433
1966	18.27061
1967	17.85778
1968	17.70717
1969	18.00949
1970	17.96672
1971	18.1999
1972	16.64965
1973	17.45166
1974	24.48091
1975	21.79679
1976	22.55319
1977	21.1284
1978	17.57412
1979	20.10688
1980	24.583
1981	24.14796
1982	23.78843
1983	21.98961
1984	23.20087
1985	21.99254
1986	16.27309
1987	15.09467
1988	14.72268
1989	15.83177
1990	16.69261
1991	15.39398
1992	14.66255
1993	13.55744
1994	13.6995
1995	14.29586
1996	15.724
1997	17.20854
1998	16.57556
1999	15.94898
2000	17.57047
2001	17.48756
2002	18.32075
2003	19.22656
2004	21.18765
2005	23.35881
2006	27.05711
2007	29.60115
2008	30.64657
2009	21.65201
2010	25.68084
2011	27.26066
2012	27.15385
2013	30.03003
2014	30.97491
2015	28.95963
2016	25.30679
2017	28.12013