Year	Percent
1960	22.104
1961	22.501
1962	22.751
1963	23.002
1964	23.256
1965	23.511
1966	23.769
1967	24.028
1968	24.289
1969	24.552
1970	24.817
1971	25.084
1972	25.353
1973	25.671
1974	26.005
1975	26.341
1976	26.681
1977	27.023
1978	27.368
1979	27.715
1980	28.066
1981	28.38
1982	28.619
1983	28.859
1984	29.101
1985	29.344
1986	29.588
1987	29.833
1988	30.08
1989	30.327
1990	30.576
1991	30.826
1992	31.077
1993	31.329
1994	31.582
1995	31.836
1996	32.092
1997	32.348
1998	32.585
1999	32.783
2000	32.982
2001	33.18
2002	33.38
2003	33.58
2004	33.781
2005	33.982
2006	34.184
2007	34.387
2008	34.59
2009	34.793
2010	34.997
2011	35.202
2012	35.408
2013	35.613
2014	35.819
2015	36.026
2016	36.234
2017	36.442