Year	Percent
1960	17.312
1961	17.835
1962	18.372
1963	18.92
1964	19.453
1965	19.964
1966	20.487
1967	21.02
1968	21.563
1969	22.115
1970	22.678
1971	23.251
1972	23.835
1973	24.427
1974	25.03
1975	25.498
1976	25.748
1977	25.999
1978	26.252
1979	26.507
1980	26.763
1981	27.021
1982	27.28
1983	27.541
1984	27.804
1985	28.067
1986	28.333
1987	28.627
1988	28.969
1989	29.312
1990	29.658
1991	30.007
1992	30.359
1993	30.711
1994	31.067
1995	31.425
1996	31.785
1997	32.147
1998	32.511
1999	32.878
2000	33.247
2001	33.617
2002	33.99
2003	34.756
2004	35.531
2005	36.311
2006	37.1
2007	34.404
2008	36.011
2009	37.645
2010	39.31
2011	41
2012	41.558
2013	42.117
2014	42.679
2015	43.245
2016	43.816
2017	44.391