Year	Percent
1960	19.226
1961	19.566
1962	19.91
1963	20.259
1964	20.613
1965	20.971
1966	21.334
1967	21.701
1968	22.073
1969	22.449
1970	22.83
1971	23.215
1972	23.606
1973	23.903
1974	23.913
1975	23.922
1976	23.933
1977	23.942
1978	23.952
1979	23.962
1980	23.973
1981	23.982
1982	23.993
1983	24.042
1984	24.212
1985	24.381
1986	24.552
1987	24.724
1988	24.896
1989	25.069
1990	25.243
1991	25.418
1992	25.593
1993	25.769
1994	25.946
1995	26.124
1996	26.303
1997	26.482
1998	26.663
1999	26.843
2000	27.025
2001	27.208
2002	27.391
2003	27.575
2004	27.76
2005	27.946
2006	28.132
2007	28.319
2008	28.507
2009	28.695
2010	28.885
2011	29.075
2012	29.266
2013	29.457
2014	29.65
2015	29.858
2016	30.082
2017	30.322