Year	Percent
1960	31.07651
1961	31.79236
1962	32.5438
1963	33.3624
1964	34.29344
1965	35.03045
1966	35.92189
1967	36.95477
1968	38.0676
1969	39.15788
1970	40.16917
1971	42.50052
1972	45.80821
1973	49.38327
1974	53.46072
1975	58.23682
1976	58.64736
1977	59.39807
1978	60.37326
1979	61.38672
1980	62.29818
1981	63.0337
1982	63.63314
1983	62.02572
1984	60.53048
1985	59.22129
1986	58.1272
1987	57.17725
1988	56.20956
1989	52.024
1990	47.81803
1991	43.59897
1992	43.1154
1993	42.52031
1994	42.03877
1995	41.79437
1996	41.85457
1997	42.13447
1998	43.67442
1999	45.0443
2000	45.95109
2001	46.25923
2002	47.62605
2003	46.2071
2004	45.13196
2005	44.58024
2006	44.80789
2007	45.50692
2008	46.27018
2009	46.5134
2010	45.88882
2011	44.36911
2012	42.35647
2013	40.37221
2014	38.88498
2015	38.08789
2016	37.9531
2017	38.33871