Year	Percent
1960	6.695
1961	6.962
1962	7.239
1963	7.527
1964	7.877
1965	8.406
1966	8.968
1967	9.564
1968	10.196
1969	10.862
1970	11.568
1971	12.313
1972	13.1
1973	13.882
1974	14.704
1975	15.566
1976	16.47
1977	17.413
1978	18.4
1979	18.592
1980	18.787
1981	18.982
1982	19.179
1983	19.377
1984	19.578
1985	19.779
1986	19.983
1987	20.187
1988	20.394
1989	20.602
1990	20.811
1991	21.023
1992	21.235
1993	21.408
1994	21.44
1995	21.473
1996	21.506
1997	21.538
1998	21.571
1999	21.604
2000	21.637
2001	21.67
2002	21.702
2003	21.735
2004	21.768
2005	21.801
2006	21.834
2007	21.868
2008	21.901
2009	21.934
2010	21.985
2011	22.054
2012	22.142
2013	22.248
2014	22.372
2015	22.515
2016	22.677
2017	22.858