Year	Percent
1960	16.203
1961	16.708
1962	17.226
1963	17.757
1964	18.299
1965	18.086
1966	17.915
1967	17.785
1968	17.656
1969	17.528
1970	17.4
1971	17.292
1972	17.184
1973	17.184
1974	17.292
1975	17.4
1976	17.46
1977	17.52
1978	17.9
1979	18.617
1980	19.358
1981	20.118
1982	20.902
1983	21.545
1984	22.203
1985	22.874
1986	23.559
1987	24.259
1988	24.974
1989	25.701
1990	26.442
1991	27.312
1992	28.2
1993	29.103
1994	30.024
1995	30.961
1996	31.916
1997	32.883
1998	33.867
1999	34.865
2000	35.877
2001	37.093
2002	38.425
2003	39.776
2004	41.144
2005	42.522
2006	43.868
2007	45.199
2008	46.539
2009	47.88
2010	49.226
2011	50.511
2012	51.765
2013	53.013
2014	54.259
2015	55.5
2016	56.736
2017	57.96