Year	Percent
1960	8.401
1961	8.684
1962	8.976
1963	9.276
1964	9.586
1965	9.904
1966	10.233
1967	10.57
1968	10.918
1969	11.275
1970	11.643
1971	12.021
1972	12.41
1973	12.809
1974	13.219
1975	13.641
1976	14.074
1977	14.517
1978	14.973
1979	15.442
1980	15.995
1981	16.562
1982	17.147
1983	17.747
1984	18.365
1985	18.997
1986	19.647
1987	20.314
1988	20.998
1989	21.088
1990	21.177
1991	21.266
1992	21.355
1993	21.444
1994	21.534
1995	21.624
1996	21.714
1997	21.805
1998	21.895
1999	21.986
2000	22.078
2001	22.169
2002	22.261
2003	22.353
2004	22.5
2005	22.703
2006	22.907
2007	23.113
2008	23.32
2009	23.528
2010	23.737
2011	23.948
2012	24.16
2013	24.373
2014	24.587
2015	24.803
2016	25.02
2017	25.25