Year	Percent
1960	15.895
1961	16.444
1962	17.008
1963	17.587
1964	18.183
1965	18.793
1966	19.419
1967	20.061
1968	20.719
1969	21.392
1970	22.081
1971	22.786
1972	23.507
1973	24.242
1974	24.994
1975	25.761
1976	26.545
1977	27.341
1978	28.153
1979	28.98
1980	29.822
1981	30.675
1982	31.544
1983	32.425
1984	33.32
1985	34.251
1986	35.496
1987	36.762
1988	38.047
1989	39.346
1990	40.661
1991	41.991
1992	42.751
1993	43.156
1994	43.563
1995	43.97
1996	44.379
1997	44.787
1998	45.197
1999	45.607
2000	46.018
2001	46.429
2002	46.841
2003	47.656
2004	48.678
2005	49.699
2006	50.721
2007	51.743
2008	52.765
2009	53.782
2010	54.796
2011	55.807
2012	56.815
2013	57.814
2014	58.786
2015	59.73
2016	60.645
2017	61.53