Year	Percent
1960	17.83969
1961	18.17054
1962	18.50863
1963	18.85299
1964	19.2044
1965	19.55987
1966	19.92257
1967	20.29183
1968	20.66894
1969	21.05062
1970	21.4404
1971	21.83806
1972	22.24526
1973	22.65775
1974	23.07848
1975	23.50612
1976	23.94248
1977	24.37913
1978	24.82765
1979	25.29257
1980	25.77851
1981	26.28229
1982	26.80916
1983	27.36018
1984	27.93997
1985	28.54569
1986	29.18437
1987	29.85452
1988	30.55585
1989	31.27956
1990	32.02888
1991	32.8026
1992	33.58381
1993	34.12624
1994	34.68244
1995	35.25183
1996	35.83656
1997	36.43279
1998	37.04972
1999	37.69046
2000	38.36089
2001	39.05713
2002	39.23829
2003	39.31432
2004	39.40465
2005	39.50531
2006	39.6197
2007	39.74677
2008	39.88695
2009	40.03597
2010	40.19655
2011	40.36747
2012	40.54992
2013	40.74029
2014	40.94063
2015	41.1508
2016	41.37087
2017	41.60113