Year	Percent
1960	17.909
1961	18.334
1962	18.748
1963	19.17
1964	19.6
1965	20.035
1966	20.478
1967	20.929
1968	21.387
1969	21.852
1970	22.292
1971	22.559
1972	22.829
1973	23.1
1974	23.374
1975	23.651
1976	23.929
1977	24.21
1978	24.493
1979	24.778
1980	25.066
1981	25.355
1982	25.611
1983	25.861
1984	26.113
1985	26.366
1986	26.621
1987	26.877
1988	27.136
1989	27.395
1990	27.656
1991	27.919
1992	28.336
1993	28.823
1994	29.316
1995	29.813
1996	30.316
1997	30.823
1998	31.335
1999	31.851
2000	32.373
2001	32.898
2002	33.766
2003	34.709
2004	35.666
2005	36.632
2006	37.61
2007	38.598
2008	39.597
2009	40.601
2010	41.616
2011	42.637
2012	43.693
2013	44.756
2014	45.826
2015	46.9
2016	47.961
2017	49.005