Year	Percent
1960	10.091
1961	10.243
1962	10.511
1963	10.785
1964	11.066
1965	11.353
1966	11.646
1967	11.946
1968	12.253
1969	12.566
1970	12.886
1971	13.213
1972	13.548
1973	13.889
1974	14.237
1975	14.593
1976	14.956
1977	15.326
1978	15.704
1979	16.089
1980	16.483
1981	16.883
1982	17.291
1983	17.708
1984	18.132
1985	18.563
1986	19.003
1987	19.451
1988	19.907
1989	20.37
1990	20.842
1991	21.215
1992	21.54
1993	21.867
1994	22.199
1995	22.534
1996	22.873
1997	23.215
1998	23.561
1999	23.91
2000	24.263
2001	24.619
2002	24.979
2003	25.343
2004	25.71
2005	26.041
2006	26.374
2007	26.709
2008	27.048
2009	27.389
2010	27.732
2011	28.079
2012	28.428
2013	28.779
2014	29.133
2015	29.49
2016	29.85
2017	30.212