Year	Percent
1960	38.47031
1961	36.75112
1962	36.29848
1963	35.83911
1964	36.08512
1965	37.02735
1966	36.84191
1967	35.56539
1968	36.04907
1969	38.56201
1970	41.66596
1971	41.20092
1972	41.25023
1973	42.0178
1974	45.98892
1975	42.22066
1976	46.62309
1977	46.66269
1978	45.43509
1979	48.32865
1980	51.10109
1981	51.92426
1982	49.30741
1983	48.32156
1984	52.02298
1985	55.08975
1986	49.85149
1987	48.23448
1988	50.43521
1989	53.25538
1990	54.31284
1991	52.8936
1992	50.56897
1993	47.17467
1994	49.247
1995	51.43374
1996	53.41475
1997	58.98905
1998	61.20657
1999	63.25536
2000	71.19146
2001	73.67254
2002	73.56534
2003	75.15508
2004	79.18828
2005	79.91447
2006	81.53704
2007	84.04037
2008	84.96098
2009	69.98285
2010	79.50355
2011	85.55501
2012	84.29479
2013	83.34327
2014	81.52668
2015	80.8622
2016	79.28489
2017	82.53965