Year	Percent
1960	20.1
1961	20.751
1962	21.418
1963	22.1
1964	22.803
1965	23.519
1966	24.252
1967	25
1968	25.815
1969	26.645
1970	27.492
1971	28.356
1972	29.238
1973	30.133
1974	31.045
1975	31.971
1976	32.575
1977	32.897
1978	33.221
1979	33.546
1980	33.874
1981	34.202
1982	34.532
1983	34.864
1984	35.198
1985	35.532
1986	35.868
1987	36.206
1988	36.545
1989	36.744
1990	36.825
1991	36.906
1992	36.987
1993	37.068
1994	37.149
1995	37.231
1996	37.312
1997	37.394
1998	37.475
1999	37.557
2000	37.639
2001	37.72
2002	37.802
2003	37.884
2004	37.966
2005	38.069
2006	38.193
2007	38.339
2008	38.506
2009	38.694
2010	38.904
2011	39.135
2012	39.388
2013	39.663
2014	39.959
2015	40.277
2016	40.618
2017	40.98