Year	Percent
1960	35.628
1961	36.175
1962	36.674
1963	37.176
1964	37.681
1965	38.188
1966	38.698
1967	39.21
1968	39.726
1969	40.242
1970	40.761
1971	41.283
1972	41.807
1973	42.757
1974	45.007
1975	47.277
1976	49.563
1977	51.843
1978	54.119
1979	56.378
1980	58.614
1981	60.809
1982	62.964
1983	63.822
1984	64.252
1985	64.678
1986	65.102
1987	65.524
1988	65.945
1989	66.361
1990	66.776
1991	67.188
1992	67.598
1993	67.792
1994	67.915
1995	68.038
1996	68.161
1997	68.283
1998	68.405
1999	68.527
2000	68.648
2001	68.769
2002	68.694
2003	68.553
2004	68.41
2005	68.268
2006	68.125
2007	67.982
2008	67.839
2009	67.695
2010	67.551
2011	67.406
2012	67.261
2013	67.137
2014	67.031
2015	66.946
2016	66.881
2017	66.836