Year	Percent
1960	27.71
1961	28.531
1962	29.461
1963	30.407
1964	31.372
1965	32.351
1966	33.347
1967	34.971
1968	36.846
1969	38.756
1970	40.704
1971	42.261
1972	43.692
1973	45.131
1974	46.579
1975	48.033
1976	49.719
1977	51.476
1978	53.233
1979	54.981
1980	56.72
1981	58.407
1982	60.061
1983	61.693
1984	63.301
1985	64.876
1986	66.679
1987	68.563
1988	70.389
1989	72.148
1990	73.844
1991	74.972
1992	75.82
1993	76.645
1994	77.452
1995	78.239
1996	78.662
1997	78.905
1998	79.145
1999	79.384
2000	79.621
2001	79.94
2002	80.299
2003	80.652
2004	81.002
2005	81.345
2006	81.528
2007	81.631
2008	81.733
2009	81.835
2010	81.936
2011	81.923
2012	81.851
2013	81.779
2014	81.707
2015	81.634
2016	81.562
2017	81.503