Year	Percent
1960	41.79122
1961	41.57497
1962	42.06281
1963	41.01488
1964	42.13464
1965	41.73852
1966	41.91655
1967	41.12737
1968	40.20111
1969	39.87962
1970	38.59161
1971	37.3321
1972	37.02887
1973	35.3214
1974	34.4508
1975	34.66049
1976	34.58034
1977	34.44596
1978	34.90544
1979	35.08477
1980	36.09656
1981	35.93623
1982	37.82368
1983	38.40147
1984	37.33939
1985	40.35458
1986	41.47765
1987	41.55799
1988	41.88157
1989	42.40894
1990	43.62401
1991	45.00273
1992	45.34201
1993	45.71249
1994	45.46645
1995	46.31058
1996	47.29627
1997	48.99464
1998	50.84996
1999	51.733
2000	51.57715
2001	52.29841
2002	52.27067
2003	52.73477
2004	52.94194
2005	53.50104
2006	54.13813
2007	54.45022
2008	53.87877
2009	55.20914
2010	55.11739
2011	55.93166
2012	56.92001
2013	57.63175
2014	57.33503
2015	58.84349
2016	59.59812
2017	59.88922