Year	Percent
1960	18.55823
1961	18.68331
1962	18.97107
1963	19.37057
1964	19.75814
1965	20.24117
1966	20.67204
1967	21.01791
1968	21.56555
1969	22.12143
1970	22.58797
1971	22.98212
1972	23.18372
1973	23.34124
1974	23.47065
1975	23.63381
1976	23.94245
1977	24.09711
1978	24.32093
1979	24.65108
1980	24.85717
1981	24.99939
1982	24.99018
1983	24.88969
1984	24.9128
1985	24.99701
1986	25.28386
1987	25.43211
1988	25.73244
1989	25.98181
1990	26.11812
1991	25.38048
1992	25.90863
1993	26.41946
1994	26.89066
1995	27.33329
1996	27.75051
1997	27.86875
1998	28.10739
1999	28.45081
2000	28.78124
2001	29.11413
2002	29.30346
2003	29.42436
2004	29.69111
2005	30.06556
2006	30.41729
2007	30.49742
2008	30.60385
2009	30.66332
2010	30.68902
2011	30.82288
2012	31.28141
2013	31.67518
2014	31.83771
2015	31.88008
2016	31.85637
2017	31.82515