Year	Percent
1960	39.50544
1961	38.95284
1962	38.07866
1963	39.11507
1964	38.7965
1965	37.15718
1966	38.0671
1967	37.27638
1968	37.02072
1969	38.43563
1970	41.00969
1971	41.51847
1972	42.09216
1973	40.93151
1974	40.88932
1975	43.0605
1976	43.10089
1977	40.58101
1978	43.1718
1979	44.52587
1980	46.55105
1981	48.89165
1982	48.5793
1983	48.1313
1984	47.69509
1985	47.91867
1986	49.95405
1987	49.74112
1988	48.64339
1989	48.55815
1990	45.56114
1991	42.7976
1992	42.93085
1993	42.35533
1994	40.21561
1995	41.54958
1996	40.43716
1997	40.80821
1998	42.80502
1999	44.19036
2000	46.75943
2001	49.3861
2002	51.12351
2003	52.03924
2004	52.37738
2005	52.63519
2006	52.68052
2007	53.50379
2008	54.5745
2009	55.89713
2010	55.61667
2011	52.90991
2012	53.19568
2013	55.34592
2014	54.47442
2015	53.19916
2016	52.90253
2017	51.73409