Year	Percent
1960	37.80069
1961	39.01317
1962	40.57309
1963	41.71254
1964	42.49755
1965	41.98327
1966	43.66611
1967	42.68673
1968	44.86691
1969	48.80028
1970	44.4876
1971	43.31118
1972	43.67714
1973	47.5312
1974	52.34193
1975	45.43735
1976	48.04819
1977	47.01198
1978	45.28457
1979	49.40199
1980	49.5948
1981	53.4999
1982	57.49757
1983	59.76599
1984	63.74084
1985	61.80323
1986	56.68586
1987	55.28533
1988	58.78874
1989	62.90405
1990	60.97135
1991	59.60175
1992	58.19583
1993	55.52281
1994	57.81587
1995	59.48767
1996	60.44278
1997	63.89677
1998	63.39727
1999	63.89872
2000	71.85759
2001	71.02037
2002	70.31477
2003	68.59609
2004	70.36997
2005	73.52123
2006	75.71114
2007	77.49084
2008	79.69598
2009	69.30978
2010	76.44846
2011	81.63555
2012	82.30584
2013	81.7303
2014	82.67205
2015	80.79436
2016	82.93778
2017	85.0533