Year	Percent
1960	3.913
1961	4.335
1962	4.8
1963	5.312
1964	5.876
1965	6.494
1966	7.157
1967	7.729
1968	8.344
1969	9.001
1970	9.705
1971	10.458
1972	11.264
1973	12.12
1974	13.033
1975	14.004
1976	15.037
1977	15.474
1978	15.803
1979	16.137
1980	16.478
1981	16.824
1982	17.176
1983	17.533
1984	17.897
1985	18.266
1986	18.641
1987	19.022
1988	19.41
1989	19.803
1990	20.202
1991	20.607
1992	21.018
1993	21.434
1994	21.857
1995	22.286
1996	22.722
1997	23.081
1998	22.949
1999	22.817
2000	22.686
2001	22.555
2002	22.425
2003	22.296
2004	22.166
2005	22.038
2006	21.91
2007	21.829
2008	22.045
2009	22.262
2010	22.48
2011	22.7
2012	22.849
2013	22.999
2014	23.149
2015	23.3
2016	23.459
2017	23.625