Year	Percent
1960	15.40338
1961	15.54924
1962	15.69757
1963	15.84826
1964	16.00157
1965	16.15633
1966	16.40252
1967	16.65321
1968	16.90912
1969	17.16888
1970	17.39925
1971	17.59044
1972	17.78451
1973	17.97986
1974	18.17816
1975	18.37876
1976	18.5582
1977	18.73883
1978	18.92183
1979	19.04167
1980	19.08383
1981	19.13375
1982	19.19165
1983	19.25559
1984	19.28937
1985	19.31905
1986	19.36056
1987	19.40616
1988	19.46175
1989	19.45164
1990	19.39251
1991	19.36528
1992	19.33775
1993	19.30522
1994	19.31209
1995	19.3283
1996	19.41143
1997	19.53362
1998	19.64761
1999	19.78108
2000	19.94446
2001	20.11143
2002	20.28896
2003	20.50412
2004	20.72544
2005	20.94492
2006	21.15604
2007	21.33651
2008	21.49198
2009	21.63273
2010	21.772
2011	21.9438
2012	22.11324
2013	22.27382
2014	22.43514
2015	22.60366
2016	22.77919
2017	22.97067