Year	Percent
1960	17.552
1961	17.929
1962	18.312
1963	18.701
1964	19.098
1965	19.499
1966	19.908
1967	20.097
1968	20.127
1969	20.157
1970	20.187
1971	20.217
1972	20.247
1973	20.277
1974	20.307
1975	20.337
1976	20.367
1977	20.504
1978	20.719
1979	20.935
1980	21.154
1981	21.373
1982	21.594
1983	21.817
1984	22.042
1985	22.268
1986	22.496
1987	22.609
1988	22.641
1989	22.672
1990	22.704
1991	22.736
1992	22.768
1993	22.8
1994	22.832
1995	22.864
1996	22.897
1997	22.927
1998	22.955
1999	22.984
2000	23.012
2001	23.041
2002	23.069
2003	23.098
2004	23.127
2005	23.155
2006	23.184
2007	23.227
2008	23.281
2009	23.335
2010	23.389
2011	23.443
2012	23.435
2013	23.382
2014	23.328
2015	23.275
2016	23.222
2017	23.169