Year	Percent
1960	19.672
1961	19.781
1962	19.89
1963	20
1964	20.11
1965	20.221
1966	20.332
1967	20.444
1968	20.556
1969	20.668
1970	20.889
1971	21.442
1972	22.006
1973	22.579
1974	23.164
1975	23.758
1976	24.365
1977	24.98
1978	25.606
1979	26.242
1980	26.791
1981	27.049
1982	27.308
1983	27.569
1984	27.832
1985	28.095
1986	28.361
1987	28.628
1988	28.897
1989	29.167
1990	29.424
1991	29.593
1992	29.763
1993	29.934
1994	30.105
1995	30.276
1996	30.449
1997	30.622
1998	30.795
1999	30.969
2000	31.386
2001	32.548
2002	33.734
2003	34.941
2004	36.169
2005	37.411
2006	38.673
2007	39.95
2008	41.242
2009	42.543
2010	43.856
2011	44.698
2012	45.445
2013	46.193
2014	46.943
2015	47.694
2016	48.448
2017	49.2