Year	Percent
1960	22.3
1961	22.523
1962	22.749
1963	22.976
1964	23.204
1965	23.434
1966	23.666
1967	23.899
1968	24.134
1969	24.37
1970	24.608
1971	24.847
1972	25.089
1973	25.331
1974	25.575
1975	25.821
1976	26.068
1977	26.317
1978	26.567
1979	26.819
1980	27.072
1981	27.327
1982	27.583
1983	27.84
1984	28.1
1985	28.513
1986	28.93
1987	29.35
1988	29.775
1989	30.202
1990	30.633
1991	31.067
1992	31.506
1993	31.946
1994	32.39
1995	32.838
1996	33.289
1997	33.742
1998	34.199
1999	34.659
2000	35.122
2001	35.587
2002	36.055
2003	36.526
2004	37
2005	37.481
2006	37.971
2007	38.47
2008	38.976
2009	39.491
2010	40.013
2011	40.544
2012	41.082
2013	41.627
2014	42.18
2015	42.74
2016	43.307
2017	43.88