Year	Percent
1960	17.06253
1961	17.4583
1962	17.86183
1963	18.18308
1964	18.44763
1965	18.71161
1966	18.97895
1967	19.24948
1968	19.52604
1969	19.80696
1970	20.09725
1971	20.39654
1972	20.70659
1973	21.02477
1974	21.35778
1975	21.73203
1976	22.12175
1977	22.52319
1978	22.94063
1979	23.37286
1980	23.82134
1981	24.28257
1982	24.76124
1983	25.03603
1984	25.16501
1985	25.29482
1986	25.4288
1987	25.56727
1988	25.71267
1989	25.86365
1990	26.02502
1991	26.12298
1992	26.18254
1993	26.25229
1994	26.33722
1995	26.43803
1996	26.55524
1997	26.68594
1998	26.83526
1999	27.00495
2000	27.19719
2001	27.41045
2002	27.49231
2003	27.48289
2004	27.47634
2005	27.46481
2006	27.44822
2007	27.42816
2008	27.40859
2009	27.39252
2010	27.38517
2011	27.41658
2012	27.47764
2013	27.54506
2014	27.62052
2015	27.70315
2016	27.7939
2017	27.89116