Year	Percent
1960	42.343
1961	44.039
1962	45.752
1963	47.475
1964	49.207
1965	50.935
1966	52.664
1967	54.386
1968	56.101
1969	57.796
1970	59.475
1971	61.06
1972	62.58
1973	64.072
1974	65.54
1975	66.978
1976	68.387
1977	69.759
1978	71.099
1979	72.403
1980	73.672
1981	74.899
1982	76.089
1983	77.24
1984	78.353
1985	79.422
1986	80.454
1987	81.446
1988	82.4
1989	82.763
1990	83.12
1991	83.472
1992	83.818
1993	84.157
1994	84.491
1995	84.82
1996	85.142
1997	85.459
1998	85.77
1999	85.885
2000	86
2001	86.118
2002	86.24
2003	86.366
2004	86.494
2005	86.627
2006	86.762
2007	86.901
2008	87.042
2009	87.187
2010	87.334
2011	87.484
2012	87.636
2013	87.79
2014	87.947
2015	88.106
2016	88.266
2017	88.429