Year	Percent
1960	55.29
1961	56.321
1962	57.162
1963	57.998
1964	58.832
1965	59.658
1966	60.48
1967	61.295
1968	62.106
1969	62.908
1970	63.704
1971	64.523
1972	65.366
1973	66.197
1974	67.019
1975	67.831
1976	68.634
1977	69.423
1978	70.202
1979	70.97
1980	71.727
1981	72.528
1982	73.373
1983	74.202
1984	75.015
1985	75.808
1986	76.567
1987	77.291
1988	78
1989	78.691
1990	79.367
1991	79.843
1992	80.128
1993	80.409
1994	80.688
1995	80.963
1996	81.223
1997	81.466
1998	81.707
1999	81.946
2000	82.183
2001	82.368
2002	82.503
2003	82.638
2004	82.772
2005	82.905
2006	83.037
2007	83.168
2008	83.299
2009	83.429
2010	83.77
2011	84.313
2012	84.841
2013	85.125
2014	85.175
2015	85.225
2016	85.275
2017	85.325