Year	Percent
1960	17.924
1961	18.032
1962	18.218
1963	18.405
1964	18.595
1965	18.785
1966	18.977
1967	19.171
1968	19.366
1969	19.562
1970	19.76
1971	19.991
1972	20.321
1973	20.653
1974	20.991
1975	21.332
1976	21.677
1977	22.026
1978	22.379
1979	22.736
1980	23.098
1981	23.417
1982	23.647
1983	23.879
1984	24.113
1985	24.348
1986	24.585
1987	24.823
1988	25.063
1989	25.305
1990	25.547
1991	25.778
1992	25.984
1993	26.191
1994	26.399
1995	26.607
1996	26.817
1997	27.028
1998	27.24
1999	27.453
2000	27.667
2001	27.918
2002	28.244
2003	28.572
2004	28.903
2005	29.235
2006	29.569
2007	29.906
2008	30.246
2009	30.587
2010	30.93
2011	31.276
2012	31.634
2013	32.003
2014	32.384
2015	32.777
2016	33.182
2017	33.6