Year	Percent
1960	17.398
1961	18.33
1962	19.594
1963	20.923
1964	22.318
1965	23.775
1966	25.297
1967	26.883
1968	28.533
1969	30.237
1970	32
1971	34.082
1972	36.231
1973	38.43
1974	40.681
1975	42.971
1976	45.295
1977	47.633
1978	49.985
1979	52.337
1980	54.682
1981	57
1982	58.372
1983	59.73
1984	61.076
1985	62.402
1986	63.711
1987	65
1988	66.412
1989	67.792
1990	69.143
1991	70.462
1992	71.749
1993	72.999
1994	73.921
1995	74.797
1996	75.654
1997	76.49
1998	77.306
1999	78.102
2000	78.879
2001	79.633
2002	80.368
2003	81.083
2004	81.778
2005	82.452
2006	83.107
2007	83.742
2008	84.359
2009	84.955
2010	85.533
2011	86.092
2012	86.634
2013	87.156
2014	87.651
2015	88.118
2016	88.559
2017	88.976